ostatus is the original fediverse. activitypub was an upgrade, that created a gap that needed bridging, very much like the gap introduced along with pump.io. mastodon eventually implemented activitypub and later on defederated from ostatus (hostile move IMHO), but IMHO neither mastodon nor activitypub define fediverse. I prefer a more expansive and, should we say, diverse understanding of the meaning of fediverse. there are some compatibility hurdles there, no doubt, but if we were to go for strict activitypub compliance, mastodon 4.0+, with not-quite-compatible "edit" feature, would likely be out