HTupolev from neogaf explains how this is possible (the gpu punching above its weight class).
"The checkerboard refers to a special sample grid that frames are rendered on; whatever resolution you want at your output, your checkerboard frame has to render half that many pixels.
Imagine rendering all of the red pixels, and then reconstructing the green ones:
For 4K checkerboard, they don't render at 1440p, they render checkerboarded images that have double the pixels of 1080p. Which means a few more pixels than 1440p, but also not arranged in a simple rectangular grid.
The checkerboard pattern lends itself to pretty good upsampling quality.
And if the sample pattern is inverted between frames (i.e. red pixels one frame, green pixels the next), it should combine well with temporal reprojection: in non-moving images this should basically allow for a perfect 4K image." - Quote HTupolev.
^ so this is how its done, if anyone was wondering.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071