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.
I understand there might be a need to develop different server to client APIs if the services that implement activity pub are very different (lemmy / mastodon). But lemmy and kbin are basically the same shit in different colours.This would be a good case to use / reuse / develop a standard API for server-client communications, I would think so.
I don’t really understand the need that kbin fills since Lemmy does most of what it does and they interact with each other. I subscribe to kbin stuff from Lemmy and Lemmy apps.
@mysoulishome I think kbin connect better to Mastodon and vice versa as well since it also have the 'Microblog' section, this post (as seen on Lemmy) and this reply are posted throught the microblog on kbin.
also I just think kbin looks nicer and more appealing to casual users rather than lemmy.