You can't use this specific syntax becauseBecause the user doesn't have a chat account yet.
I just tried searching their username in the list of all chat users and got nothing:
Only users who already have chat accounts can be superpinged just by using @@<username>
. For a user who has a site account but not yet an account on Chat.SE, you need to superping them using the <userID>@<site>
syntax.