There have been bugs in Lean that allowed people to prove False, from which you can prove anything (they have been fixed).
Otherwise, if you check that no custom axiom has been used (via print axioms), the proof is valid.
It's easy to construct such an example: Prove that for all a, b, c and n between 3 and 10^5, a^n=b^n+c^n has no solution.
The unmeaningful proof would enumerate all ~10^20 cases and proof them individually. The meaningful (and probably even shorter) proof would derive this from Fermat's theorem after proving that one.
Otherwise, if you check that no custom axiom has been used (via print axioms), the proof is valid.
It's easy to construct such an example: Prove that for all a, b, c and n between 3 and 10^5, a^n=b^n+c^n has no solution. The unmeaningful proof would enumerate all ~10^20 cases and proof them individually. The meaningful (and probably even shorter) proof would derive this from Fermat's theorem after proving that one.