I think that mathematical proofs, as they are actually written, rely on natural language and on a large amount of implicit shared knowledge. They are not formalized in the Principia Mathematica sense, and they are even further from the syntax required by modern theorem provers. Even the most rigorous proofs such as those in Bourbaki are not directly translatable into a fully formal system.

I haven’t read the paper yet, but I’d imagine the issue is converting the natural language generated by the reasoner into a form where a formal verifier can be applied.

such high performance program indeed could potentially be superior, if it would exist (this area is very undeveloped, there is no existing distributed well established solution which could handle large domain) and math would be formalized in that program's dsl, which also didn't happen yet.