I’ve read the first issue of Journey into Mystery in which Thor appears, and he doesn’t spout the pseudo-renaissance talk from the get-go. But it doesn’t talk long. And of course it’s easy to fanwank; he doesn’t initially remember his Asgardian life and mannerisms because Big Daddy Odin’s spell took a while to wear off.
I believe he was exploring an ancient tomb which - no matter who your brother is - would have to raise the odds of you encountering a demon-haunted gem.
I imagine Cyttorak’s the sort of entity who would see “likely to pick fights with the most powerful psychic on the planet” as a point in Cain’s favour, too.