Being banned from Twitter is a consequence of your speech, but it's also restricting your freedom to speak. I imagine Musk feels that not restricting people's freedom of speech is more important than the consequence of banning them.
That doesn't mean he thinks there should be no consequences for speaking. Just that the consequences shouldn't limit your freedom to speak.
In this case, people fired from SpaceX are still free to speak out about Musk's brand and its influence on SpaceX. Their speech has not been restricted.
The people banned from Twitter are still free to speak about whatever as well. Frankly I think SpaceX's actions limit speech more than a Twitter ban. Ex-Twitter users just have to find a new platform, at their convenience. Fired employees have just had there livelihood taken away and have to drop everything and find something new before their savings run out.
To be clear I think SpaceX was well within it's rights to fire these people, but as consequences go I see firing as far more consequential than a Twitter ban.
I'd expect them to go on a interview campaign after this actually. I think they'll be quite publicized and amplified.
People who are fired from SpaceX have absolutely no access to the SpaceX communication channels they were fired for exercising the wrong kind of speech on.
Obviously, I am not saying what these employees are saying is equivalent, but cancelling somebody for what they say is not always cancel culture.
Edit: Isn't getting kicked off twitter simply a consequence of saying something that's against twitter's TOS? What definition of free speech is Musk using?