Phil Spencer pretty much explains this fully in the Giant Bomb interview (thanks
@mpg1 for the linking this originally) starting at around the 30:30 mark. He even mentions the Titan XP!
Found it to be a good interview.
They built a machine that can take XO games (at XO settings) and run them at native 4k, and 900P games by all accounts also if devs really want to put the extra work into it. At same but more stable framerate.
But would people really prefer for most games to run at native 4k at XO settings or, non native but with higher quality settings?
I remember when it was first announced, and they had to clarify that it wasn't mandated within a couple days, because of the fallout by people thinking dev's would be forced to use the power to do native 4k.
Now there's fallout because dev's are choosing to use the power via CBR, even though so far it seems to be 2160cb full framebuffer, which I also don't expect to last forever.
Even if AC:Origins could run at native 4k, do people really want XO settings just at 4k? Also their engine is built around CBR, and has been for a while now, just the way the industry is going.
They built a machine that can run XO games at native 4k, thank god dev's aren't doing that in lots of cases.
Edit:
@Johnglen I know you really hate CB image quality. This wasn't directed directly at you, as you would probably prefer the native 4k with XO settings. I suspect you would be in the minority though, and it's the way engine development is going hopefully it will reach the IQ that is good enough for you sooner rather than later.