Is he seriously tweeting about this? Basically that is insubordination and grounds for termination. If this manager doesn't agree with the company policy, and isn't willing to pick up the slack of workers who are leaving or who have left and maintain the release date of the product, he's going to be shown the door too. Spending time creating a map of who left isn't a good move either and is not going to give any excuses as to why work can't get done, especially if it wasn't asked for by senior leadership.

Every one is replaceable, including this manager who is questioning senior leadership. Complaining about it on Twitter isn't going to change anyone's mind who has made these decisions, it's just going to result in your own dismissal. It's a lot better to quiet quit and look for a new job if you feel your job has become impossible.