It's not a public forum, it's a privately owned social media website/app...

The owners can kick anyone out they want.

Musk knew that, but apparently didn't know why the old owners kicked them out, it's because the vast majority of advertisers and users don't want them their.

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.