There are no instances in ActivityPub either. There's only actors.
The instance'ification came outside of the work on the spec, though it did start with the particular shape of how sharedInbox was defined. I wish I had pushed harder for sharedInbox to retain the actor-like properties I believed were important, rather than the implicit-routing that Mastodon pushed for. My biggest regret.
Nonetheless, even then, there's still no "instance" in AP, in terms of the spec. In terms of the fediverse, the way it became, that's a different story.
@cwebber Aren't there servers though? The two sound rather equivalent to me in a protocol that is not message-centric and tightly binds actors with the server the hosts them (rather than purely some cryptographic key).
