Per the rules in their link, the clock starts after publication in a qualifying peer-reviewed journal so, no, I don't think that's the reason.

https://www.claymath.org/wp-content/uploads/2022/03/millenni...

I'm sure they've been getting a lot of press inquiries and don't want anyone to misinterpret their silence as refusing to engage with OpenAI's solution.

Now they have a statement on the record that can be quoted with a gentle reminder of the qualifying criteria (which makes clear they have nothing to evaluate either way yet).

Also to emphasize the original optimistic ideals of the prizes and their role as neutral arbiters who aren't going to litigate anything outside the scope of the prize itself.