It's not impossible, but I also know of no instances of an AI being told to prove something in Lean and exploiting such tricks. Surely Anthropic also had some agents looking for issues with the generated proofs. Personally, I am also comfortable trusting Kevin Buzzard, who was leading the human team aiming to formalize FLT and discussed this a bit on his blog.
Finally, the good thing about Lean is that if anyone ever finds a new compiler bug (which, by the way, are being searched for extensively using AI), you can correct the bug and recompile any old proofs of which you are suspicious. Any tricks in a false proof must be exploiting a bug in the Lean compiler, so as we increase trust in the compiler over time we also increase trust in every previously compiled proof.
I agree it's not a 100% guarantee, but in this case the human proof is well-understood and written about by many experts, so I think Claude had plenty of material to work with. Even if the task was enormous, I don't think any of the individual steps are out of the scope of what we have seen from current AI tools.