They use the same protocol, but so far both of them have only implemented the server-to-server (federation) portion. Neither (to my knowledge) have implemented the client to server interactions.
Lemmy has its own API, which is completely separate from the ActivityPub spec. Kbin doesn’t have an API yet, but one is actively being developed.
Nice. I had a similar idea bouncing around in my head, but didn’t get around to implementing it.
The main hurdle for me was that there’s no way to interact with the content (i.e., upvote, boost, comment, post.)
I can’t see a way around this without either a private instance, or creating an account on each respective platform - this is a limitation of ActivityPub IMO. I’m curious as to your thoughts?