There are a lot of architecture astronauts that will keep talking about “we should develop a grand-unifying system”, but most of them are all talk and little result.
When push comes to shove, the developers that actually ship are the ones that look at problem they want to solve and work on that.
I think the most interesting opportunity here is that ATProto’s strength is that it was built for a “credible exit”, i.e, it allows users to migrate from any centralized network to a decentralized one.
IOW, it would be a lot easier to implement Fediverser on top of ATProto than of ActivityPub.