This is not an interview process for hiring a junior dev at FAANG.
If you're Sam & Greg, and Satya gives you an offer to run your own operation with essentially unlimited funding and the ability to bring over your team, then you can decide immediately. There is no real lower bound of how fast it could happen.
Why would they have been able to decide so quickly? Probably because they prioritize the ability to bring over the entire team as fast as possible, and even though they could raise a lot of money in a new company, that still takes time, and they view it as critically important to hire over the new team as fast as possible (within days) that they accept whatever downsides there may be to being a subsidiary of Microsoft.
This is what happens when principles see opportunity and are unencumbered by bureaucratic checks. They can move very fast.
I don't know anything about how executives get hired. But supposedly this all happened between Friday night and Monday morning. This isn't a simple situation; surely one man working through the weekend can't decide to set up a new division, and appoint two poached executives to head it up, without consulting lawyers and other colleagues. I mean, surely they'd need to go into Altman and Brockman's contracts with OpenAI, to check that the hiring is even legal?
That's why I think this has been brewing for at least a week.