There are many instances of Mastodon, and due to its federated nature, you can use any of them to access it, and even host your own.

What's stopping them from just blocking them all and continuing to block new ones?

Nothing is stopping them, but like most things in blocking free speech, it’s a game of cat and mouse.

The long tail is very long

It's not that long. You could probably these servers with an automated process.

Sure, but if you have an account on a different server, you can still see things posted on mastodon.social if you have followed someone there.

That’s the context I was missing I think.