This is just false I think. I'd have to search for it, but in some of the recent stories about how morale at Meta is at an all-time low due to the layoffs and fiefdom-building I recall seeing a number of current and former Meta employees comment.
Plus, my guess is that the vast, vast majority of Facebook employees just aren't privy to outright illegal acts, and most people aren't going to break their nondisparagement clause just to break the latest bitchy thing somebody said regarding Zuck.