I regret that comment, which was less polite than I aim to be on here. Let me try to write something a bit more substantial.
I don't think "discover all math that can possibly be discovered" is really coherent. Even if one conceptualizes mathematics as "answering well-posed open problems," historically resolutions to such have tended to raise more questions--I would argue that there are now more interesting open questions than there have been at any time in the past, despite (in fact because of!) the fact that there are more mathematicians than ever, resolving more problems than ever.
At any fixed capability level I think it is likely we will see lots of problems remain open, including many basic open problems we know about at present. It's much easier to pose an interesting question than to answer one; it seems to me that the difficulty of interesting questions we can generate is basically unbounded.
But also mathematics consists of much more than this--less verifiable tasks include things like "understand such-and-such an object," or "find a cool phenomenon," or "develop a theory," though such tasks are often benchmarked by their impact on problem-solving. I think AI will eventually (perhaps soon) be able to do these kinds of things but the idea that it will exhaust the supply of math, or that we won't want to develop human capital to understand some portion of what is discovered, seems to rely on an understanding of math and our motivations for working on it that is, at least, alien to me.