You know how annoying it is when you receive a paper to review, and it's just badly done or even AI slop? And you're like "it's not my job to clean up the stuff you were too lazy to do"? Now also imagine that it's about a topic that you've been working on for years. And that you can't really trust the results or check them, but it still takes away a large part of the rewards you could get for doing it well. And that slopocalypes happens to your entire field, all at once. I can understand why the mathematicians are pissed.