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.
It’s entirely up to the developers to collaborate and implement according to the spec though. ActivityPub wasn’t really designed with this use-case in mind, so the spec has limitations that warrant a specialized API, or extensions to the AP spec.
For example, the outbox is an OrderedCollection. This is how you would list posts in a magazine/community. However, there’s no way to sort by other metrics such as hot, top, or trending. Even Mastodon has its own REST API.
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?
A command-line Fediverse client that doesn't require a server (github.com)