Well I can't seem to find his comment. Look at his profile page and it's not showing up, I tried to reopen this thread and it shows 32 comments. But mine and yours are the only one showing up.
Well, I'm not really sure about the comment numbers, etc. I saw a pull request solving negative comments in communities, though. So I guess it's still being worked on.
You can tag other users with @[email protected] when using the official web UI. I don't know about 3rd party clients, though.