Check out the DF analysis on those*), both say the end result is 2160c (i.e. checkerboarded 4K - checkerboarding in and of itself doesn't mean any target resolution, it's just a reconstruction technique, for example ME:A is 1800c on a Pro).
The way checkerboarding works is that you'll end up rendering all of the vertical pixels and half of the horizontal pixels, then reconstruct the image from there. Consider a display resolution of any arbitrary size. Now divide it into black and white tiles in a checkerboard pattern (it doesn't matter if the tiles are 1x1 pixels, 2x2 pixels, 4x4 pixels, whatnot, as long as it divides evenly for both axis). If you render either colour of tiles, you'll end up rendering all of the vertical axis and half of the horizontal axis. A contrieved example:
Code:
Pass 1 Pass 2
xx|oo oo|xx
oo|xx xx|oo
xx|oo oo|xx
oo|xx xx|oo
Think "xx" as the pixels being rendered, and "oo" as the pixels reconstructed. As you can see, all of the vertical resolution and half of the horizontal resolution is rendered in any given pass. Barring any upscaling techniques (which DF would surely mention), to say a game is rendering at $RESOLUTION checkerboarded means that there are $RESOLUTION_Y and $RESOLUTION_X/2 pixels rendered in every pass. But that reconstruction, that's the tricky bit, which is why the results vary so much.
(having said that, given how blurry the results are in RDR2, I wouldn't be surprised to learn that it's just rendering a regular image at 1920x2160 and stretching the result horizontally to 2160p)
*) Edit: here for example is the
DF analysis on GOW - at some point they actually compare native 4K and checkerboarded 4K (using Rise of the Tomb Rider on One X)
