From Jeff's answer to the deleted question* linked to from the question you link to:
Your name is inferred from your openid until you specify a display name. You have no display name specified.
Thus your inferred username is ...
~quack
... because it's based on your openid URL. It's not an actual, valid display name.
If you check unutbu's page there's no name specified.
* visible only to 10K+ meta users.
