Proofs that cannot be grasped are like unproven truths. Worth very little... They only stand through the credibility of the inventor. It downgrades the mathematician to a politician.

If it were a giant Coq term then sure, you can ignore those pesky details like "understandability".

I would not agree with that. One can do math for personal pleasure.

You can draw all the squiggly lines on paper that you want, but it won't mean anything until someone else interpets it. You cannot prove logic that only exists in your own head, because you cannot prove yourself rational. Your very existence does not even make sense without other people perceiving you.

Math in the form of a research community which has built up centuries of work for each other to further build on? Absolutely.

The proof can be completely fine, but if no one can understand it then it won't be accepted.

Except that, from reading the article, it sounds like he doesn't much care about proving his result to the community. Don't make the mistake of assigning the motives and values of a founder to a late-career tenured mathematician. According to the article, he doesn't care for travel. It's not a stretch to conclude that he doesn't care to communicate his proof to other mathematicians.

I'm guessing he found an argument that was sufficient to convince him (and only him), put it online as a slight concession to his colleagues, and then kicked his feet back.

Surely everyone else would benefit from his having a co-author, but it sounds like he can't be bothered, and doesn't care if no one else understands his ideas.