Their protocol (called the “Authenticated Transfer Protocol” or “AT Protocol” for short) fixes some issues that ActivityPub has.

For example, you can use your own domain name as your handle, even if you’re not hosting the server yourself. This makes your username portable - you can move to a different server but keep the same username. You just need to update some DNS records. It’s not possible to move server but keep the same username with ActivityPub, since your username is always at the server’s domain name.

What I’m hoping is that the developers of ActivityPub take some ideas onboard, but it’ll be hard given the current design of the protocol. Maybe we’ll get a new version of ActivityPub one day.