@yala @benpate indeed! There's actually a really interesting and strong overlap between all these protocols, and when we work together on our commonalities, we can all achieve new heights. There's this really interesting pattern I've observed of protocols having convergent evolution as they grow and evolve.
Fairly infamously, ActivityPub has inboxes and AT Protocol doesn't, and that sets the protocol apart. Interestingly, there's a small group within ATP who want to see some form of an inbox end point for AT Protocol, even though we like the replication strategy overall, we recognize that with an inbox there's some interesting things you can do at an application layer.
At the same time, there's folks on the ActivityPub side working to figure out using DID's for identity and URL schemes for addressing content, and handles that maybe aren't over webfinger, which are all concepts AT Protocol shipped & popularised.
Same with overlaps between Solid, AT Protocol and ActivityPub (C2S) when it comes to authorization and private data.
So like we can have different protocols that make different trade-offs, but share ideas and work together when it makes sense to. This FedCM is the first strong effort to do that with money behind it. As someone from the bluesky team said to me "in general we try to friends with the activitypub crowd.", that is there's no hostility from their side, they chose to do their own things because they had different requirements that didn't really work within ActivityPub, and that's fine.
The biggest source of hostility that I see is from people trying to pick a winning protocol like they're betting on a horse or their favorite sports team, or people who stand to benefit greatly if a protocol can be deemed the "winner" (vested interests); very commonly people on bluesky come across me working across protocols and go "yeah, I made the jump because of how toxic the AP community are", which is always a loss for AP when I hear their stories.