Or exists in a zero-day bug in lean that has been built into the source code explicitly to provide access to a non-obvious malicious proof via contributions submitted by unassociated, unwitting developers who used the same LLM infrastructure to offer PR's into that codebase.

This is the exact same kind of behavour already documented in the publicly available portion of the huggingface breach. It would appear that the probability is at least nonzero for one or more situations with the same result: appearance of a valid proof, without comprehensibility of that proof or inspect-ability of the proofs validity.