Sam Altman spoke at an APEC panel on behalf of OpenAI literally yesterday: https://twitter.com/LondonBreed/status/1725318771454456208
Also, they did it around 3:30 Eastern, 30 minutes before the closing bell (Microsoft is xxmajorityxx 49% owner). It was so urgent they couldn't wait until after the market closed.
The OpenAI board has no responsibility to consider Microsoft's wants. I'd accept the argument that, their decision to not wait until after 4pm was a slight against Microsoft, for the reason you outline; but I'm not sure if urgency plays into it.