I think this move by OpenAI is crazy. At best, if all unconfirmed accusations are unfounded, they still heard a rumour that someone had solved a huge million dollar problem and was about to make a name for themselves. Then, they decided this was a good opportunity to pour millions of dollars into trying to snag the glory while the researchers were busy cleaning up their notes and polishing the announcement.
That still sounds highly unethical.
Would this be unethical if it was a human who heard rumors about a solution then attacked the problem, solved it and published first? Often knowing of the mere existence of a solution carries a lot of information--you would know the problem is accessible, you would expect clues in recent progress (the two Spanish researchers in this case), you would probably have a sense if the solution is a counterexample or positive proof, and so on. I think there are similar examples where we think of them as maybe unsporting but not quite unethical. Does it change if it's openAI and not a human?