To answer the question a different way, I think you are asking how we know the proof actually matches the description the human provided? And I'd say we can't know for sure, but the idea is that you can pretty concisely write and check yourself that the problem is accurate, i.e. "There are an infinite number of primes" or whatever, and then even if an LLM goes off and makes up a lean proof wildly different from your description, if lean says the proof is valid then you have proven the original statement. I guess in theory the actual proof could be way different than what you thought it would be, but ultimately all the logic will still check out.