I personally don't see how jokingly saying a character from a completely different company/universe is in a Marvel movie is a spoiler. I especially don't see how that makes me an asshole.
Furthermore, I don't see how saying Stan Lee is in Infinity War, who makes a cameo in every single Marvel movie, is a spoiler / fake spoiler.
The only people who would have the movie "spoiled" are those who have no understanding of the Marvel universe.