Yea, but that’s just a lowest common denominator (e.g. it doesn’t include things like lemmy community sidebars), and also generally not appropriate for a client application. ActivityPub transmitts all events that are happening (posts, likes …) between servers, and they are supposed to index and aggregate things (e.g. sum up votes, sort posts). It’s just not feasible to expect the same from a mobile app for example, you’d have to at least create another standard for that.
So services end up implementing their own client APIs to fit their needs. And imo that’s actually a good thing, because it allows them to try out features and specialize on different use cases. But afaik the ActivityPub people are working on another standard for client APIs, at least it’s on their radar.