I'm genuinely confused why so many people are reacting so quickly to this news like it's the end of Mastodon. We can't conclude anything just by virtue of the fact that he signed an NDA. We don't know the terms of the NDA. It could simply be that he can't talk about Meta's specific plans.
More to the point, as the originator of the network and the one in charge of the source code, I feel like it's his responsibility to be informed of what companies like Meta are planning to do. If an NDA is the price of admission to that knowledge, and provided that the terms aren't egregious, he should go.