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.
Shrug. I'm part of the "plus other stuff" fediverse and have had nomadic identity in the fediverse since long before either ActivityPub or Bluesky existed. I have it today. I already had it for years when Mastodon first arrived in the old StatusNet (plus other stuff) fediverse. I'm semi-retired but migrating that work to ActivityPub in my spare time. Whoever told you that we've been unable to do so is in fact spreading uninformed bullshit.
@mikedev that's definitely the BlueSky folks talking about why the AT Protocol is advantageous.
Yes, they were mostly talking about Mastodon, but their blog post did seem to hide the fact that anything ActivityPub enriched with other protocols will most certainly empower nomadic identity.