I'm pretty sure it considers the 360 game still a 360 game. If it was considered an Xbox One game, then you wouldn't need emulation to play it.
"The 360 games think they're running on the 360 OS, which they are. And the 360 OS thinks its running on the hardware, which it's not, it's running on an emulated VM. On the other side, the Xbox One thinks it's a game. That's why things like streaming, game DVR, and screenshots all work, because it thinks there's just one big game called 360."
I am not saying it's not emulation, it is. But the Xbox One still consider it has a Xbox One game, that why you can use Xbox One features for screenshot or DVR.