From Jeff's answer to the [deleted question][1]* 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.

[1]: https://meta.stackoverflow.com/questions/25542/display-names-bugs-no-allowed-cant-reset-unset-kills-profile-copy