Not to snatch the prize, but:

1. to test their new model

2. to be able to say "you came with the proof, but our model can do this too"

3. to verify the result. This is also a great thing for the math.

Of course, it makes sense to test your new model on the problem that is solvable at all, but not solvable by you just yet. It makes no sense trying to test your model by throwing resources into an unsolvable problem.

Well, it turns out the rumors were incorrect, NS was not solved by other guys, and OpenAI became the first one.