That could simply mean that he disagreed with the outcome and is expressing that disagreement by quitting.
EDIT: Derp. I was reading the note he wrote to OpenAI staff. The tweet itself says "After learning today's news" -- still ambiguous as to when and where he learned the news.
Probably a similar situation.
Since he was removed as Chairman at the same time as Altman was as CEO, presumably he was excluded from that part of the meeting (which may have been the whole meeting) for the same reason as Altman would have been.
just base logic.
It is:
> As a part of this transition, Greg Brockman will be stepping down as chairman of the board and will remain in his role at the company, reporting to the CEO.
https://openai.com/blog/openai-announces-leadership-transiti...
The portion you quoted says he will remain at the company. This post is about him quitting, and no longer remaining with the company.