That diversity of applications under development can be seen on the ActivityPub Application Watchlist (which is in turn input to Fediverse Party). Also implementing ActivityPub as an interoperability protocol has merit all on its own, even when not federating with the broader Fediverse (but instead between single application instances, or within a restricted domain). See for more info on this: Positioning ActivityPub: De-Emphasize “Being Part of the Fediverse” as primary USP.
Alternatively or in addition to what @how proposes, you might add a compressed paragraph that conveys this concept, and follow with “… the remainder of this paper will focus primarily on Microblogging, provided by Pleroma and Mastodon […]” or something like that. Pleroma is interesting, as it implements the full AP specification with Client-to-Server API (C2S) support as well, where Mastodon has a custom API for this.