It's that it's not broken, it's just not implemented as part of the comment autocomplete.
While there is a related requestrelated request to get that working, as I noted in my answer there the current feeling is that adding the list of editors to the autocomplete would introduce unnecessary complexityunnecessary complexity. @reply
ing to an editor without the assistance of the autocomplete will still work, however.
In your particular case, your original comment referenced @omg_ponies
which isn't a valid match for that user's name (the underscore cannot be used as a replacement for the space), so no notification would have been generated.