“Consequences for thee, not for me” feels pretty hypocritical. In this case Musk wants the consequences for speech be limited to things (like being fired) that he doesn’t have to worry about because he is rich.
If you're an employee at spacex and Musk uses internal communications to say something you don't like, you can leave spacex, which would be a consequence for musk's actions. So your point doesn't hold. He is not immune from the consequences of his speech in the exact same scenario.
A job is little more than a business relationship where a person agrees to do labor in exchange for money. Either side of that relationship has the ability to terminate that relationship as a consequence of speech they might not like.
The better approach is to form a union, in order to address the colossal power imbalance between SpaceX's executive committee and the people who do the actual work. It's likely you'll get fired for that as well, but it's better than leaving in "protest". Elon Musk probably spends 50x more time thinking about his hair plugs than he does about engineers departing his companies.
Either way, to act like this "business relationship" is perfectly reciprocal is either naive or malintentioned.
I agree, unionization at face value seems to be a great tool to empower workers. As for reciprocity I don't think anyone is suggesting it's perfectly balanced. If you're using that phrasing as a device to suggest it's extremely unbalanced then I would wonder what data you're using to come to that conclusion. It is, after all an entirely voluntary relationship being formed in a country with no shortage of jobs.
