In ActivityPub, posts, comments, and users themselves are identified by URLs consisting of DNS names and small sequential IDs, with the same entity having a different ID at each instance it is federated to. For example, the comment I'm replying to is ID 6283426 on its home instance, and 5909380 on my instance, and 5405408 on the home instance of the community this thread is in.

In ATP (bluesky's protocol) everything is identified by cryptographic hashes and digital signatures, while the DNS-based URL of a user's current "personal data server" at the time they created a post is not part of the post's identity.

The difference in data models is a major impediment to bridging the two protocols. If two different bridges convert the same post (or other entity) from either one of these protocols to the other, they will always be creating duplicates.

I'm not an expert on either protocol but it seems to me that the only way to bridge them in a way that works well would be for both protocols to be substantially modified for the specific purpose of interoperating with each other, and so far I haven't seen any indication that either side is interested in doing that.

They won't. Blue skies federation is partial at best from everything I've heard. And apparently federation is mostly about client-side interface. With things on the server side, being much more centralized and heavy on the server itself. Specifically for algorithm tuning and commercialization. Two goals that are dimetically opposed with what mastodon wants to achieve. I'm not saying that no one will try. I'm just saying that it won't work well then blue sky has no interest in it.

The only way such a thing happens is if Mastodon just flat out takes over and it is a last ditch attempt to stay relevant in some way for blue sky. Because outside of a situation like that. It would make being subject to an algorithm and advertising major negatives. When you could just go to anywhere else on the network and get the same content without either of those.

I think in many instances threads federating makes a lot more sense. They ultimately want a lot of the same things. But it isn't their only product and only chance at a payday. Meta gets a lot of money from other sources. And I suspect they're playing this as part of a long game since they can tie it back into other established services of theirs. Like Instagram. Where Even if someone on Mastodon shares or links to stuff on Instagram, they still get to harvest data and possibly sell advertisement.