The effort from going to a paper proof, even one that everyone agrees is correct, to a formally verified one is still gigantic. Requiring all submissions in maths papers to come with a verified Lean proof would be a complete non-starter - most mathematicians don't even use Lean.
In many cases, the problem isn't that the statement of the theorem can't be properly expressed in Lean. It's very easy to write down the statement of FLT in Lean, for example. But for theorems whose proofs have been elusive for decades or centuries, the proofs very likely include objects in rather advanced mathematics that are not at all easy to encode in Lean. This is one of the current struggles of the FLT formalisation process, for example.
In any case, the problem in this case has nothing to do with formalisation, it's simply a case of a bad submission that for unclear reasons didn't get properly peer reviewed.
In many cases, the problem isn't that the statement of the theorem can't be properly expressed in Lean. It's very easy to write down the statement of FLT in Lean, for example. But for theorems whose proofs have been elusive for decades or centuries, the proofs very likely include objects in rather advanced mathematics that are not at all easy to encode in Lean. This is one of the current struggles of the FLT formalisation process, for example.
In any case, the problem in this case has nothing to do with formalisation, it's simply a case of a bad submission that for unclear reasons didn't get properly peer reviewed.