Comment by SkidanovAlex

It is the latter. If you are certain your theorem is stated correctly, and you believe that the Lean kernel against which you validate is correct, your proof is correct.

This is how the theorem for FLT looks in the particular proof we discuss here:

theorem fermat_last_theorem (n : ℕ) (hn : 3 ≤ n) (a b c : ℕ) (ha : 0 < a) (hb : 0 < b) (hc : 0 < c) : a ^ n + b ^ n ≠ c ^ n

As long as this statement is correct, and the kernel is correct, the proof could be trillion lines of code, and if the kernel says it is correct, it is correct.

This proof was checked against TWO independently built kernels. So you would need TWO kernels to have the same bug to mistakenly accept an incorrect proof.

(Not impossible: such a bug indeed was recently discovered (and patched))