I saw a talk by the peole working on FrontPage last week, they use the Bluesky Relay, filter it down to only see the posts they're interested it, and have their own AppView. It doesn't yet have the same kind of intereoperability with Bluesky that Lemmy does with (say) Mastodon; FrontPage posts are only visible to FrontPage. But, there are discussions on how to get beyond that.
No, because the AT Protocol is not designed for interoperability, but rather for entrenching the silo owned by the main node (BlueSky) whilst giving the illusion of being decentralised. It’s to decentralised social media what Microsoft’s OOXML file format (tl;dr: a memory dump of Microsoft Word’s internal data structures encoded in XML, and useless to anything that’s not Microsoft Word or a very precise emulation thereof) is to open document formats.
Both of these services appear to be dependent on BlueSky. I.e., if BlueSky ceased existing, or cut them off from its API, they’d die. In that way, they’re not that different from “Log in with Facebook” or similar.
One could theoretically make one’s own independent AT Protocol network, but not in a way that interoperates with BlueSky as a peer. You’re either a subsidiary part of its network or you don’t exist as far as it’s concerned, which is a much poorer value proposition than ActivityPub and related protocols.