I think the whole story is that a rumour started about Anthropic solving a millennium problem and OpenAI went in full panic mode trying to beat them to it.

Turns out an anthropic employee was a coauthor on the original paper, but that was something he was doing on the side, not under the Anthropic's institution (using several LLMs, including OpenAI's and Anthropic's).

Allegedly, OpenAI then tried to coerce the original author into authoring their proof and cutting off the anthropic guy.

These dicks just wanted to be the only ones involved and it backfired.