Only feels last minute to those outside. I've seen some of these go down in smaller companies and it's a lot like bankruptcy - slowly, then all at once.
Everything points towards this being last minute both for people outside and people inside. Microsoft caught with their pants down, announcement before markets closed rather than just waiting a bit, and so on.
Announcing something huge like this before market close is not something that can be interpreted as anything other than either a huge timing mistake or a massive feeling of urgency
I find it hard to believe that the board of OpenAI isn't smart, savvy and self-interested enough to know that not delaying the announcement by an hour or so is the wrong move. That leads me to believe that yes, this was something big and worthy enough of being announced with that timing, and that it was probably not a mistake.
They also said Greg was going to stay at the company and then he immediately quit. I find it very hard to believe that smart, savvy, and self interested are adjectives that apply to a board who doesn't know what their own chairman thinks.
Even smart, savvy, and self interested people can't always predict what individual humans are going to do. It's certainly an interesting wrinkle, but I don't think it's relevant to the limited scope of the analysis I've presented here.