Wasn't it stated that Devs have only had the X1X Dev kit for like a month. So you are seeing demos that were created for the conference running stable 4K 30FPS with a month using the Dev kit with probably no optimization of their code. Yes they are using Checkerboarding because the main mid gen system they probably had was the PS4 Pro and they needed to convert that code to the X1X because the checkerboarding is not the same between the 2 consoles. If anything this proves out Phil statement that it takes nothing for Devs to upgrade their game to 4K to support the X1 with their current game engines. It will be interesting to see if these games can go true 4K on X1X once the developers get to their optimization phase.
On another level, I wonder if MS will show during gamescon Gears, Horizon and Halo 5 on X1X hardware in public viewing. MS was showing Gears and Horizon behind close doors but they really need to get those franchise stables in public running in 4K with 4K assets. Icing on the cake and a good marketing tool is if they can have all those games running at 60FPS. This would give more justification to the X1X price and hardware and be a good sell for MS.