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.