I feel like there's a difference between a spoiler and a fan theory that happens to be right? Because like, for the former, that's someone straight up telling you what's going to happen, and that's pretty annoying. But for the latter, usually anyone who's invested enough in the story to guess the plot twist ahead of time will have more of a reaction of, "AHA! I correctly put together all the clues! I'm so smart!", like they just solved a mystery. At least that's my experience with it.
But yeah, my general response to fan theories, correct or not, is "hm yes, that sure is an interesting observation" 