This is a very predictable consequence of criticizing your employer via a public letter. Criticize internally all you want, and influence the changes that you want to see happen. Employment is a two-way relationship. If you don't like your employer, you are free to leave. If they don't like you, they are free to fire you (within legal bounds). Publishing an openly critical letter and signing it is a quick way to get…
They critized Musk, factually. Musk is not their employer, SpaceX is, but you just proved their point since you believe Musk and SpaceX are one and the same.
It is the board that should be holding him accountable, but they aren't. Most boards today are lame ducks to collect a paycheck. No one is challenging him because he is delivering results (although I would argue it's actually Gwynne Shotwell keeping the shipping sailing). So there's that.