What “different question”? If you’re just trying to argue that the ability of private entities to legally restrict the speech of users on platforms that they provide is a (legal and constitutional) limitation on free expression, well, yes it is. Also, water is wet.
What you haven’t done is to make any kind of convincing argument that it would be constitutional or beneficial to take away that ability from private entities. (The issue of whether there should be more and smaller private entities controlling such platforms rather than a very few very big ones is a separate, although related, matter.)
Yeah, both of those are about the oligopoly issue, not the core issue of whether private entities are entitled to restrict speech on their platforms.