I think he wants to have group chat added to the private messaging, ie. so that you could invite other people to join in on the private conversation you are having with someone else and make it a group/conference chat -> they could see the chat history you've already had in that conversation and join in on the discussion and drop out as they see fit.
Shoutbox with groups wouldn't quite fill this hole because it could be important that the private message history is saved and I don't see the shoutbox doing that securely, it's not it's original function. Shoutbox is a shoutbox - it's meant for a few shouts and random comments, it's not meant to be the complete chat implementation.
But I think having a well done group conversation added to private messaging would require a complete rewrite of the existing private messaging implementation.
I also agree that it needs to be done eventually to get Nodebb up to the bar, but it's probably a lot of work and not a fast patch job.