O talento de Humphrey é, na verdade, saber como e onde encontrar as respostas para as perguntas, em vez da onisciência real. Eu li mais ou menos na série como você fez, mas parece que o passado e os poderes de Humfrey foram abordados em mais detalhe mais adiante na série:
In Heaven Cent, it is implied that Humfrey's actual magical talent may be no more than that of locating other magic that may be of use to him. Though probably not magical talents, he does have extreme curiosity and a (usually) very good memory, which through the course of his life led him to become a master of information, as he recounted in Question Quest. Further, he recounted that he had believed that his native magical talents were not magician-class, and that he attended Demon University and gained a degree in magic, so that he could deserve the Magician appellation by which he had already become known.
De certa forma, não é muito diferente de como o talento de Bink emprega intermediários, ao invés de simplesmente fazer com que a mágica contra ele "fracasse".