Como Thanos foi capaz de derrotar Hulk na Infinity War sem usar nenhuma pedra?

Thanos não usou a power stone no Hulk .

Thanos é um ser massivamente poderoso sozinho, mesmo sem as pedras. Note que a Power Stone não estava brilhando na época e também Ebony Maw deixou Hulk e Thanos lutarem sem qualquer interferência, dizendo:

No, let him have his fun...

Para citar uma fonte oficial:

Escritores da Guerra Infinita Dizem Que Thanos Poderia Ter Batido Hulk, Power Stone ou Não , em Movieweb :

Avengers: Infinity War has brought up more than a few debates, but screenwriters Christopher Markus and Stephen McFeely have now revealed that Thanos would've beat the Hulk even if he didn't have the Infinity Gauntlet with the Power Stone, fueling more debate. The opening scene takes place on the Asgardian refugee ship after Thanos and his Black Order have taken it over to locate the Tesseract. In a last ditch effort to defeat Thanos, Loki sends the Hulk into a rage to fight. However, the fight doesn't go in the Hulk's favor.

Stephen McFeely believes that the Mad Titan would've destroyed the Hulk with his own bare hands in Infinity War. This would explain why Ebony Maw allows the fight to continue and makes the scene look like Thanos was just having some fun, like a cat playing with a mouse. Looking back at the scene, Maw and Thanos are pretty calm, even when it appears that the Hulk has the upper hand in the battle.

McFeely had this to say when asked if Thanos could still beat Hulk without help. I think he could. I may be speaking out of turn, but I think Thanos could kick the Hulk's ass without the (Power Stone).

O artigo explica ainda:

However, the opening Infinity War scene wasn't just to show that Thanos could beat the Hulk. Stephen McFeely and Christopher Markus wanted to establish the fact that Thanos is the biggest villain in the entire MCU. Marvel Studios boss Kevin Feige revealed early on that the first 5 minutes of Infinity War would show off the immense power of the Mad Titan and he was not kidding around.

McFeely explains, We want to announce Thanos as the biggest villain in the MCU, so he takes out the previous reigning champion. And by defeating Hulk relatively easily and savagely, hopefully there's a sense of dread over the course of the rest of the movie for anyone else who comes up against him.

Thanos é strong o suficiente mesmo sem a Power Stone ou qualquer outra pedra infinita. Existem várias evidências que sugerem que,

Cena 1:

No filme de guerra Infinity Avengers, durante a batalha em Titan, onde Iron Man e Thanos lutam um contra um, Iron Man usa seus nonobots para se certificar de que Thanos não pode fechar o punho e ativar as pedras infinitas. Então ele lança uma dúzia de mísseis em Thanos e Thanos fica ileso após a explosão.

Cena 2:

No filme Guardiões da Galáxia, quando Ronan decidiu trair Thanos, Korath diz isso a ele,

Master! You cannot! Thanos is the most powerful being in the universe.

Eu discordo strongmente com o Hulk sendo a teoria Loki aqui. Considere os seguintes pontos,

1. Aqui está o que os irmãos Russo disseram sobre Banner incapaz de se transformar em Hulk,

So we thought an interesting direction to take him in is what if Banner, who typically uses the Hulk to solve crisis situations, what if the Hulk were no longer interested in solving those problems for Banner? So that the relationship is becoming increasingly dysfunctional. That’s what’s going on here. People have speculated whether there was some fear on the Hulk’s part about having to face Thanos again. But I think ultimately what it is, is that he’s tired of playing hero to Bruce Banner.

2. Banner tenta convocar Hulk duas vezes nos filmes em que ninguém está assistindo, uma vez quando ele estava na suíte do Hulk Buster. Se ele era Loki, por que ele faria isso?
3. Banner ao falar com Tony tem conhecimento sobre eventos ocorridos durante o Age of Ultron quando ele diz:

Tony, you lost another super-bot?

e

This suit's already kicked the crap outta the Hulk.

Ele sabe como usar a suíte Hulk buster que ele ajudou a projetar.

Referências:

1. Guardiões da Galáxia - Transcrição
2. Russo irmãos sobre o Hulk