That's something I have wondered, too. What could cause NP to shut up.
Certainly people have tried to contact him and ask what happened. Someone would have leaked the conversation if it happened. So, I conclude that he has actually STFU, as hard as that is to imagine. How would you get him to STFU? It's in his self-interest to STFU, such as a huge monetary penalty if he doesn't honor the NDA. A severance agreement might include some form of NDA; however, this appears to be a broader silence than that would entail, and includes everyone and everything. Hopefully we wake up one morning and it turns out to be for a good reason.