I think the idea of the Fediverse is in constant flux, and that is part of its appeal to me. Originally the term applied to the OStatus protocol rather than ActivityPub.
And while ActivityPub is indeed the dominant format today it would, in my mind, convey a smaller space than the wider potential.
If it had been decided Fedi can never be more than OStatus the term would be weaker.
I do believe my diagram can be used as an explanatory model whichever conviction one has.