This is a logical fallacy and the equivalent of saying 'you are welcome to practice free speech, in this here sound-proofed room'.
Twitter is one of a small handful of platforms where an individual can share an idea and have that idea spread - as long as the owners of Twitter don't disagree with that idea. It's not because twitter is special, it's because it was one of the first to achieve a sufficiently large userbase.
Defending arbitrary censorship on these platforms as 'oh well it's a private entity so they can do what they like' misses the forest for the trees. Technology has shifted the power balance for free expression, and applying pre-technology laws and mindsets to it just empowers that small handful of individuals to manipulate public discourse even more. Twitter doesn't quite have a monopoly on speech, but it's damn close in terms of practical outcomes. The fact that the legal definition of `monopoly` hasn't caught up with that, doesn't change the matter.