Doesn't even have to be a surprise. Pretty much startup employment agreement in existence gives the company ("at the board's sole discretion") the right to repurchase your shares upon termination of employment. OpenAI's PPUs are worth $0 until they become profitable. Guess which right they'll choose to exercise if you don't sign the NDA?
You are thinking of going up against an entity worth billions of dollars and somehow winning? That doesn't happen outside of movies. You'll accept whatever they decide, or end up like those Boeing whistleblowers if you make yourself enough of a nuisance.
One particularly sus term in my employment agreement is that I adhere to all corporate policies. Guess how many of those there are, how often they're updated, and if I've ever read them!