“ActivityPub’s API is how client applications interact with the data on a user’s main account server. It lets the user read data on the same or other servers, and it lets them create activities and other kinds of objects on that server that get shared (under the user’s control) with the rest of the world.”

I can't see how Apub's C2S API can realistically be implemented. It's fairly light on details and if I'm understanding it correctly the only standard way to get activity from the server is to pull from an actor's inbox, which has to be an OrderedCollection of all the activity the actor has received (likes, notifications, posts, the lot). This shifts a lot of the work to clients which, apart from being being very classist, is very limiting for implementations.