In my experience they write an informal proof first, check it without Lean, and only then turn it into Lean code. That last step sometimes takes them much longer than checking their proof without Lean, which is a problem humans have as well. What they're truly bad at, however, is explaining their proofs. Until they improve at that, we'll have to keep resorting to Lean to check whether they solved something correctly.

I'm less sure about the following, however: I think that while current LLMs are bad at explaining their own proofs, some models are great for discussing older, human-written proofs. For this use case, from best to worst, Gemini > Claude > GPT-Astra >>>>>>>>>>>> GPT-Sol.