but how would a person on a single-user fediverse instance be any more censored than a NOSTR user?

I think to put my answer more simply, yes nostr is still more censorship resistant. Because lets say lemmy.world bans your hypothetical single user instance.

Now everyone on lemmy.world is banned from reading your content.

In nostr lemmy.world the client cannot stop it's users from requesting your content from other relays.

They do not have the power to block thousands of users from accessing your content.