Can you specify what leverage you think Brubeck has over an independent professor's career in order to make threats?
I see none, and consequently Brubeck's explanation makes more sense to me. I understand he meant these words, which he supposedly retracted on the spot, in a "why would you ruin your career with this behavior / turning down the opportunity I am offering" way.
You don't think that is the likely explanation?
I'm not sure the value of speculating, but since you insist...
If someone said to you "you're going to regret this, mo-fo!", do you really need to suppose they had a specific plan in mind, rather than just intent to intimidate?
My best guess would be that he was threatening for OpenAI to publish without giving any credit to Buckmaster (who has been working on Euler overall for at least 10 years). Obviously this would be absurd, but no more absurd than that Brubeck was demanding - that Levant was excluded for any credit and could not have him name on any OpenAI writeup (with this being the exact sticking point that Buckmaster was refusing to accept).
The funny thing is that this threat, as is often the case, seems to say more about the insecurities of the person making the threat than the person they are directing it to.