Using double-@ user identifiers for actor discovery is not an ActivityPub feature. It's a bastardization of webfinger resource search added and popularized by Mastodon.
In ActivityPub URLs are the identifiers, be it for actors or anything else. And the URL can be just of a domain, therefore it's perfectly possible to have domain named identifiers.
In ActivityPub URLs are the identifiers, be it for actors or anything else. And the URL can be just of a domain, therefore it's perfectly possible to have domain named identifiers.