The whole idea of BlueSky supporting nomadic identities but the rest of the ActivityPub (plus other stuff) Fediverse being unable to do so is such an oversold idea.
A new service using ActivityPub behind the scenes (and not the AT Protocol) can absolutely support nomadic identities, even if the service doesn't treat a whole website as the actor.
It will still use did:plc, same as AT Protocol (BlueSky), but once done so, an application that understands how to work with did:plc can dereference an actor based on the DID.
That said, an existing service will simply not be compatible with this idea, without changing how it operates.
