Comment by digikata

To borrow some definitions from Systems engineering for verification and validation, this question is one of validation. Verification is performed by Lean and spec syntax and logic enforcement. But Validation is a question of is if the Lean spec encodes a true representation of the problem statement (was the right thing specced). Validation at highest levels is probably an irreplaceable human activity.

Also, on the verification side - there could also be a window of failure that Lean itself has a hidden bug in it too. And with automated systems that seek correctness, it is slightly elevated that some missed crack of a bug becomes exploited in the dev-check-dev loop run by the AI.