essentially, DIDWG and SWICG had a joint meeting in 2021 to formalize that DID would use as:alsoKnownAs (and that the official activitystreams context would adopt as:alsoKnownAs). this led to the situation we currently have, where the property is in the activitystreams namespace but is actually defined in the DID spec. and of course, to further complicate things, mastodon had already been using the as:alsoKnownAs property unofficially for some years prior… with a slightly different semantic usage. where the DID spec defines it as “different identifiers for the same subject”, mastodon’s usage is more like “different subjects with the same controller”. i really wish that mastodon had gone with some other method of establishing requirements for a Move activity, such as rel-me links in the actor’s attachment fields.
a DID subject doesn’t have to be an actor, btw, it’s just the thing that a URI would be identifying, and the corresponding resource would be describing.