TL; DR: Tarkin está plenamente ciente de que havia Jedi - ele lutou ao lado deles nas Guerras Clônicas. Mas o Império estava desesperado para apagar os Jedi da memória coletiva da galáxia e investiu uma enorme quantidade de tempo e energia para fazer isso acontecer. Foi em grande parte eficaz.
Nota: Itálico em todas as citações é do original; negrito é meu.
Problemas com a questão:
Tarkin nunca diz ou faz qualquer coisa que sugira que ele não acredita na Força, ou não sabe sobre os Jedi.
The Jedi are extinct, their fire has gone out of the universe. You, my friend, are all that's left of their religion.
Aqui, ele está simplesmente dizendo "Vader, os Jedi estão todos mortos, e você é a única pessoa que usa a Força". Isto é verdade, até onde ele sabe - e ele sabe o que sabe porque ele ajudou a exterminar os Jedi . E ele não está muito longe da verdade - quando ele apresenta essa linha, há apenas dois Jedi (Obi-Wan e Yoda) em uma galáxia habitada por quatrilhões de pessoas - e ele não sabe que nenhum desses Jedi ainda está Vivo, porque ambos estão escondidos há 20 anos.
E Motti não está dizendo: "Os Jedi são um mito e a Força não existe".
Don't try to frighten us with your sorcerer's ways, Lord Vader. Your sad devotion to that ancient religion has not helped you conjure up the stolen data tapes, or given you clairvoyance enough to find the rebels' hidden fortress...
Ele está basicamente dizendo: "Os Jedi não existem mais, e a Força é estúpida. A Estrela da Morte é muito melhor do que qualquer coisa que você possa fazer com a Força".
A galáxia como um todo e a classificação e arquivo imperial:
No romance Canon da Disney Guerra nas Estrelas: Estrelas Perdidas , dois cadetes imperiais, os melhores de sua classe na mais prestigiada Academia Imperial da galáxia, sabem pouco ou nada sobre os Jedi, apesar da fato de que a Ordem 66 só foi promulgada cerca de 10 anos antes:
Yet the Imperial Palace was one of the grandest and most elegant structures on the entire planet; apparently it had once been a temple of some kind.
- Star Wars: Lost Stars
O "templo de algum tipo" que o personagem está descrevendo, agora o palácio imperial, é o antigo Templo Jedi em Coruscant. Foi um templo Jedi por milhares de anos, até 10 anos antes.
Mesmo antes, apenas alguns anos depois da Ordem 66, a galáxia já havia deixado de acreditar na Força, e os Jedi pareciam nada mais do que um mito:
So far as Ciena could tell from the few holos she’d ever been able to watch, most people in the galaxy no longer believed in the Force, the energy that allowed people to become one with the universe. Even she sometimes wondered whether there could ever have been such a thing as a Jedi Knight. The amazing tales the elders told of valiant heroes with lightsabers, who could bend minds, levitate objects— surely those were only stories.
- ibid
Mais tarde, o outro estudante - que desde então se tornou um oficial, abandonou o Império depois de testemunhar a destruição de Alderaan - junta-se à Rebelião, mas ainda considera Jedi e a Força como superstições bobas. Isso é notável, considerando o fato de que ele está falando com Dak, que realmente conhece Luke:
It was all Thane could do not to groan. Please, not more superstitious nonsense about the “Force.” In his opinion, the rebel troops needed to be motivated by the harsh truth about the Empire, not crazy religious beliefs.
- ibid
Assim, a tentativa do Império de esconder o fato de que os Jedi já existiram foi extremamente bem-sucedida, e esse sucesso veio rapidamente.
Grand Moff Tarkin:
Quanto ao próprio Tarkin, podemos nos voltar para o romance Canon da Disney Star Wars: Tarkin . Tarkin lutou sob e ao lado de Jedi - incluindo Anakin Skywalker e Obi-Wan Kenobi - durante as Guerras Clônicas. Ele conhece a história deles. Ele suspeita que Vader e Palpatine são Sith. E ele até sabe - ou é quase certo - que Anakin se tornou Vader.
Tarkin realized that the Emperor’s current residence had once been the headquarters for the Jedi — though practically all that remained of the Order’s elegant Temple complex was its copse of five skyscraping spires, now the pinnacle of a sprawling amalgam of blockish edifaces with sloping façades.
- Star Wars: Tarkin
Tarkin não só sabe que os Jedi existiram - ele também suspeita que Vader e o Imperador são Sith, e se pergunta se Palpatine mentiu sobre as circunstâncias em que seu rosto foi deformado e Mace Windu (e outros três Mestres Jedi) morreram:
There were many stories about what had occurred that day in the chancellor’s office. The official explanation was that members of the Jedi Order had turned up to arrest Supreme Chancellor Palpatine, and a ferocious duel had ensued. The matter of precisely how the Jedi had been killed or the Emperor’s face deformed had never been settled to everyone’s satisfaction, and so Tarkin had his private thoughts about the Emperor, as well. That he and Vader were kindred spirits suggested that both of them might be Sith. Tarkin often wondered if that wasn’t the actual reason Palpatine had been targeted for arrest or assassination by the Jedi.
- ibid
Mesmo para Tarkin - que conhecia muitos Jedi - os Jedi parecem ser um mito, apenas 5 anos depois da Ordem 66; ele também começa a descobrir quem realmente é Vader:
That look again, Tarkin thought — or at least that suggestion of a look that always made him feel as if Vader knew him from some previous life.
“We no longer speak of the Jedi,” Mas Amedda had said when they had watched Vader issue his warnings to members of Coruscant’s underworld. It struck Tarkin now that the Chagrian’s attitude wasn’t one that was confined to the Emperor’s court. In the five short years since the Order had been eradicated— Jedi Masters, Jedi Knights, and Jedi Padawans wiped out by the very clone troopers they had commanded and fought beside— the Jedi already seemed a distant memory.
- ibid
A profundidade de seu conhecimento dos Jedi também é revelada:
Tarkin had respected the Jedi as peacekeepers, but as generals they had proven failures. The Jedi Master with whom he had served most closely during the Clone Wars was Even Piell, to whom Tarkin’s cruiser had been assigned. Brusque and bellicose, the Lannik excelled in lightsaber combat, seeming to have integrated every possible fighting style, but he, too, had his flaws as a strategist. If Piell had deferred to Tarkin during their mission to investigate a hyperlane shortcut into Separatist-held space, they might have avoided capture and imprisonment, and perhaps the Lannik would have survived at least until the end of the war.
The Force had endowed the Jedi with wondrous powers, but their biggest failing was in not having used the Force in all ways possible to bring the war to a quick end. By remaining faithful to their ethical code, they had allowed the war to drag on and spiral downward into a meaningless bloodbath. The conflict’s sudden conclusion and the Order’s decision to depose Supreme Chancellor Palpatine. had taken nearly everyone by surprise. But Tarkin suspected that even if the Jedi had restrained themselves from rising against Palpatine in his moment of glory, the esoteric Order had doomed itself to extinction. Where their flame had burned bright for a thousand generations, technological might was the new standard.
- ibid
Tarkin já estava bem ciente da afinidade de Vader com pessoas que sufocam a Força, mais de uma década antes do incidente a bordo da Estrela da Morte:
Tarkin knew merely by the gasping sounds that began to erupt from the Sugi that Vader was performing that thumb-and-forefinger dark magic of his to crush the crime lord’s windpipe.
- ibid
E, finalmente, ele descobriu - embora não tivesse certeza - quem realmente era Vader:
Tarkin didn’t always agree with Vader’s methods for dealing with those who opposed the Empire, but he held the Dark Lord in high esteem, and he hoped Vader felt the same toward him. Very early on in their partnership — soon after both had been introduced to the secret mobile battle station — Tarkin grew convinced that Vader knew him much better than he let on, and that behind the bulging lenses of his face mask, whatever remained of Vader’s human eyes regarded him with clear recognition. More than anything else it was those initial feelings that had provided Tarkin with his first suspicion as to Vader’s identity. Later, observing the rapport the Dark Lord shared with the stormtroopers who supported him, and the technique he displayed in wielding his crimson lightsaber, Tarkin grew more and more convinced that his suspicions were right.
Vader might very well be Jedi Knight Anakin Skywalker, whom Tarkin had fought beside during the Clone Wars, and for whom he had developed a grudging appreciation.
- ibid
O Imperador estava ciente das suspeitas de Tarkin, mas não os via como um problema, porque Tarkin claramente não se importava de ser liderado por dois Siths:
Sidious assumed that Tarkin had puzzled out that Vader had once been Anakin Skywalker, under whom Tarkin had served during the war. Tarkin may also have determined that Vader was a Sith. If so, it followed that he accepted that Sidious was Vader’s dark side Master. But Tarkin’s intuitions were important only in the sense that he never revealed them and never allowed them to interfere with his own ambitions.
- ibid
O Imperador há muito procurava promover uma amizade entre Anakin e Tarkin, mas não funcionou até que Anakin se tornasse Vader:
During the Clone Wars, Sidious had made every attempt to promote a rapport between Skywalker and Tarkin, but the relationship had never prospered to his satisfaction. Then came that business with Skywalker’s Togruta apprentice, Ahsoka Tano, which, while it had provoked further disaffection in Skywalker, had also created a rift between him and Tarkin that perhaps had yet to mend. Yes, they had partnered since the end of the war, but— to Sidious’s own annoyance— absent a true appreciation for each other’s talents.
- ibid
Por que Palpatine tentou encobrir a existência dos Jedi:
Porque enquanto as pessoas acreditassem na Força, e soubessem o verdadeiro valor dos Jedi, a Força poderia se esforçar e restaurar o equilíbrio - destruindo Palpatine, o Império e os Sith:
Darth Plagueis had once remarked that “the Force can strike back.” The death of a star didn’t necessarily curtail its light, and indeed Sidious could see evidence of that sometimes even in Vader — the barest flicker of persistent light. Attacks like the one directed against Tarkin’s moon base [where the Death Star was under construction] and discoveries like the one on Murkhana were distractions to his ultimate goal of making certain that the Force could not strike back, and that whatever faint light of hope remained could be snuffed out for good.
- ibid