ActivityPub is a successor to ActivityStreams/Salmon which Mastodon and GNU social are currently using. At least with these it's not really a xkcd/927 situation.
Why do you drop other names? Diaspora is a tool, not a standard. If they are still developing that then they may actually implement to read and share data in the standard as well.
However, they haven't done that yet. So they're competing "standards," even if one isn't standardized.
An announced successor is still xkcd/927 until it actually replaces all use of the predecessors.
Case in point: pump.io is the successor of OStatus and StatusNet.