According to https://en.wikipedia.org/wiki/Fediverse:
The majority of Fediverse platforms are based on free and open-source software, and create connections between servers using the ActivityPub protocol. Some software still supports older federation protocols as well, such as OStatus, the Diaspora protocol and Zot. Diaspora* is the only actively developed software project classified under the original definition of Fediverse that does not support ActivityPub.[5][6]
You would have to look at those citations to see how authoritative they are. This may also still be open to interpretation?
That's fair. But what they've said so far seems to strongly point at this being the reason.