Let’s differentiate between a Fediverse service, like kbin or Calckey, and something like Fedilab, which is a mobile app.

Fedilab supporting 8 different services is different from any one service being interoperable with 7 others. This is because the mobile apps aren’t using ActivityPub to talk to the different servers, they’re using each service’s API. And they don’t all have the same API, or support the same feature set.

Consider Lemmy and kbin. Lemmy doesn’t have support for thr microblogging feed, so an app that supported both would either need special support for the microblog on kbin, or exclude it.

Then say they want to roll out Mastodon and Calckey support. Well, Mastodon is a relatively simple microblog in many respects, with posts, and lists, and favouritrs, but Calckey has a “Channels” feature, emoji reactions, instant messaging, “Antennae” saved searches, local-only posting, and advanced Markup features. Everything that makes Calckey different from Mastodon just gets hidden if special development isn’t done to actually support Calckey features.

And then there’s Pleroma and Akkoma, which have many similar features, but different implementations, and so on, and so forth.