He's trying to say that your "groundbreaking" hashtag idea is so similar to IRC that some people will be thinking you're being ironic.
And obviously it doesn't have to be a # character.
All that aside, parent is right, you're basically re-inventing IRC. A more user-friendly client may be a good idea, but it's something that's really hard to make money on, so it's unlikely anyone would bother working on it.
Anyway, as someone else let me now, kik intruduced exactly this feature with the exact same limit on users in one chat already.