The fact that the press release is 50% dedicated to repeating that OpenAI is supposed to be a non-profit and help all of humanity isn't enough for you to believe this is the reason?
The abruptness of the firing and the fact that they give his lying to the board as the reason is why I don't believe that this is over a general disagreement on direction.
It's exactly the other way around - if they dismiss him for a vague general reason, they're much less exposed to litigation than they would be if they falsely accused him of lying.
Them leaving does not imply accusations are false. They may like him, they may dislike new boss regardless of accusations, they may dislike overall future direction. They may think they would be fired some times later regardless.