No, UPNP has not been implemented and it is not guaranteed to work either. It might help in some cases though. The clients communicate between themselves and do not handle game connections via the server - that's why people need to be able to connect to each other directly. For that the some kind of NAT is needed (the game does not have any bypassing procedures implemented as far as I know). Usually it requires people to forward an UDP port from their router to their game client. If at least one player is unable to connect to all others the game won't start.
Some routers might use other techniques and do not require manual configuration, others can be set to DMZ temporarily to run checks.
Makes perfect sense but that would mean for every pair of players in the room, one of the two should have their UDP port open, right?
So there can only ever be 1 person in any room with their port closed?
I'm pretty sure I've been in rooms that ended up playing with multiple people not having set-up their port-forwarding.