It'll be fine. They'll be on two pretty arbitrary public ports, but it works just fine if you have a vaguely sane nat implementation in your router. That's the whole point of nat punching. And even if you have an incredibly broken nat implementation that won't accept UDP packets from other sources than the server you originally connected to, there's a fallback pathway at the first layer of that port 3074 protocol that bridges p2p messages through live's backend.

As another example, last time I checked WebRTC didn't use UPnP, but has all the same issues as a fundamentally P2P UDP protocol. They use STUN/ICE/TURN, live uses custom protocols filling the same niches.