"Don't say this thing, or we'll ban you forever from the platform."
If I walk up to you and say, "Don't say this thing, or me and my 9 associates here will punch you in the face," that's compulsion. Period.
Chomsky said it best: "If you’re really in favor of free speech, then you’re in favor of freedom of speech for precisely the views you despise. Otherwise, you’re not in favor of free speech.”
And yes, Twitter, under the first amendment, is free to associate with whomever they want (or to refuse to associate with them)! Twitter isn't compelling Meghan Murphy to do anything anymore than than twitter is, at this moment, compelling me to give them my wallet.
On the other hand, if you said "Give me your wallet, or me and my 9 associates here will punch you in the face," you'd be committing a crime.
That's the difference between being kicked off twitter and punching someone in the face. One of them is a crime. No matter how purely you uphold the principle of free speech, it doesn't make sense to compare someone who doesn't share those values to a criminal.