I'll believe that there is an emulator when I see it. From the info we have now and the fact that you have to download the whole game with a pretty small library of games it is clear for me that they merely ported 360 games to the X1 with the DVD as the license key. I really doubt there is any emulating involved.
I'm assuming it works the same way that the 360 works to emulate Xbox games.
It's not an actual emulator, but instead each game has an emulation profile that needs to be downloaded/or is already on the HDD.