First off, you're wrong about Urbit - Urbit explicitly does not have the address space to give each human their own planet, believing that it's a political ideal that not all humans deserve the same rights. (It might be true that each human currently using Urbit does have their own planet, but the stated long-term goal of Urbit is for this not to be the case.)

But even if you were right about Urbit, it wouldn't change my claim. I understand your claim. I am not trying to claim that Mastodon is just like Urbit. My point is that sometimes, in Mastodon, there is no difference between an instance and a user, just like you claim about Urbit. Yes, sometimes, there is, but Mastodon's norms had to figure out how to handle the case where there is no difference, because that case exists. And yes, when people collectively refuse to peer with an instance, the user(s) on that instance are not stripped of their posts and do not have their identity taken away, because those things are under the Mastodon instance's control... but it would be pretty misleading to say "If you set up your own Mastodon, you can't get banned."

And there's no reason to expect that Urbit, if it grows to the level of popularity as Mastodon, won't develop analogous norms.

I still don't really understand the difference, is it not possible to host a service on your planet, and then have people make accounts on it and log into it with some other means? If so, that would seem to make it identical to mastodon, or any other internet service really. There doesn't seem to be any reason you can't bridge it with random HTTP services. I mean, in theory you could even run a mastodon-like service inside a planet?