Is this intentional?
As I understand, reverting “should be as easy” as changing the anon-username and anon-email to new values (of course, the original values are not stored anywhere, but we might know them by means external to discourse), and then sending a “generate password” message to that email. This seems like something that an admin should definitely be able to do.
Similarly, an admin could allow a new user to “take the role of” an old anonymized user, in exactly the same way. I see no reason to explicitly disallow an admin to do such thing (as far as I can see, this is now “explicitly and intentionally disallowed”, since if anonymizing were implemented as just a change of email, name and login-credentials, everything would be the same, but this “account restoring” would be possible by just changing these fields).