It sounds like he hasn't verified the results of a problem that he has personally worked on, so how many of these problems have actually been verified?
Incorrect. The statement in Lean can itself be wrong. Moreover, they could be exploiting a kernel bug in Lean, of which we had one published literally a week ago.
Besides for what others have mentioned, the lean proof could be proving something else. Given AI’s propensity to hallucinate, seems like someone should check the lean proof actually expresses what it’s claimed to.