Hi @julian,
the document is a report from @evanprodromou mid 2024, not a standard and not part of #AP. And at step 4 it reads "4. Optionally:". But it proposes a solution - thanks for bearing with me. So the #rfc7033 subject: "acct:alice@correct.example.com" would be it.
Feels hacky, IMO the profile should be explicit about the handle just like the way round #webfinger is explicit about the actor id.