If Meta had put Threads on the AT protocol instead of ActivityPub, things could have been so different... maybe there's still time. The engineering philosophy for AT / Bluesky is incredible, being able to develop your own timeline algorithms, distributed moderation agencies, etc. But whenever I give Bluesky a try unfortunately I just can't find people talking about my interests. This seems like a scale problem, which Threads solves.
Nothing is stopping you from building your own AP algorithm either though, right? An "algorithm" is, ultimately, a ranking model that ranks a set of incoming posts based on some criteria no? Nothing stops someone from building their own ranking model for themselves.
I agree that the Bluesky userbase doesn't really talk about the topics I care about and I think the audience is a big factor.
Threads indeed could have used AT protocol to bootstrap Threads audence, but it's likely that a corporate sponsor would not want their brand associated with Bluesky's audience.
Anything on AP or AT can bridge both ways anyway
Meta's ActivityPub is very fake, it's not only off by default and functionally limited, but also has a dark pattern to get you turn it off.
It would be the same if they do ATP.