In a strictly legal sense, you can be allowed to call your neighbor ugly. There will be no legal consequences, because of a law/constitutional amendment protecting free speech.

On a personal level, however, your neighbor might not like being called ugly, and retaliate by avoiding you or insulting you back. This is a consequence, but not a legal one.

I think, Musk view is that expression on Twitter should play a "legal guardian" type role in moderating content on the site, as opposed to say blocking negative content (and you could argue that as a site that makes money selling ads, blocking negative content could be the smart play, similar to the NYT not hiring idiots to write for them), but that the SpaceX employees, when fired by their employer, are facing consequences not on a legal but a personal level.

Of course, there's a very good chance this is just backwards rationalizing the erratic, irrational behavior of a emotionally unstable person.