OpenAI's statement implies he was aware of the demotion... but his statement seems to imply he wasn't.
I guess the most likely situation is that they put out the press release, told him (or vice versa) and it took him a bit to decide to quit.
Maybe my experience with corporate communications is different, but all it implies to me is that he was not removed as President and was being permitted to stay on under the new CEO.
Its totally fair (your interpretation) to think that. He was removed as chairman though, which IMO is a demotion. I think its disingenuous on the part of OpenAI _unless_ originally Greg said he was ok with the new terms. If a company says "Person X will remain as President and report to the CEO" you would think they have worked it out with person X _before_ announcing it.