On a closer look, however, DID spec and Fediverse seem to be aligned. In the DID spec, alsoKnownAs links different identifiers related to the same subject (a person, organization, etc). In Fediverse, it links different actors. It is more accurate to think of entity (person, organization) represented by actor as the subject. The controller of actor is, in fact, the server.
Consequently, the usage of alsoKnownAs in FEP-c390 also makes the most sense.