While Americans seem obsessed with defining "free speech" as being exactly the same thing as the 1st Amendment, it might help to ask exactly why the 1st Amendment is a good idea.

JS Mill argued that censorship is wrong on a rights basis, that it leads to bad ideas being unchallenged, and it leads to people not being able to understand the reason why ideas are rejected. If censorship by a government is harmful, how is deplatforming by a near-monopoly different? I suspect some left-wingers argue this in a hypocritical way, because they think it will "trigger the conservatives" to force them to attack the actions of private companies, but this is hardly a good argument.