Something interesting is going on here. If you log in to Plus, you can use New Hangouts _right now_. And they _do_ interoperate with XMPP users _right now_ -- I just tried it.
So does this mean the rollout is not complete, and what they rolled out as Hangouts today is not what they'll ultimately roll out? Or does it mean -- as I suspect -- that New Hangouts is XMPP under the hood, but they plan to flip the 'interoperate' switch to 'off' once they think nobody will notice?