What are you saying, then? That sub-900p will be the norm on Xbox multiplats?
He's probably saying that when an console exclusive game is made (be it PS4 or XB1) it's a shit ton easier to hit 1080p because you are working with a single, locked, set of hardware that can be mined for performance improvements.
That is not the case when you're working across three different platforms, each with their own unique set of challenges. Then it suddenly becomes super duper hard to pull off a stunt like that.
We've been super lucky so far with 1080/60 because almost all games have been cross-gen ports, and therefore the added power in the current gen allows developers to reach that level.
That is going to change when the Xbox 360 is no longer the minimum spec, but the XB1 is instead. Then developers will be pushing the hardware in new ways, and framerate/resolution will be the victim of that. It's exactly the same way as with the last generation. With the exception being first party /exclusive stuff where the approach is different.