It’s no more public than a shopping mall. The mall and Twitter are owned by a corporation or private entity so they can kick you out for any reason. They own the space, not the government. A public space is somewhere typically owned by the government like the library, town hall, roadway, or park. A common misconception is that anywhere people can freely enter is a public space, that’s not really how it works. If you think I’m wrong, go wave a Nazi flag and cause a disturbance at a mall and see how fast they kick you out.

In what sense is it public? It’s owned by X and no one else.

People want to think it’s a public forum because a lot of people use it. But that doesn’t actually make it public.