So you're saying they wouldn't go for 1080p if they could do it with little other stuff turned off.
Of course they would, they're Crytek. Push everything to the limit and watch the hardware break down.
Why is this so difficult to understand?
If they could go to 1080p with no performance hit that would mean they were making a game at 900p and intentionally leaving a huge amount of power unused. That would be absolutely stupid.
According to Crytek, they made a choice that 900p (god I hate that term) was the optimal resolution to allow them to achieve what they want to achieve. Because fewer pixels means they have more time to spend on each pixel.
That is not just true for Xbone, it is true for PS4, PS3, 360, Wii U, and basically anything in use today.