If I read them right, they're basically claiming that all those extra megapickles of the D800 just get you larger files, but they don't get you any sharper prints.
I can believe that without trouble.
Consider all those who've compared 1.6x cropped images from the 5DIII with the 7D and found the 5DIII to be the clear winner. That's the exact same type of comparison, only with an even more dramatic difference in mepickle count.
If DXO's numbers are are all meaningful, then they should discover the 1DX absolutely trouncing the D800 on their sharpies-per-megapickle score. If they don't discover that, then this is nothing more than yet another bizarro-world imaginary number pulled from their collective netherbits.
I gotta say, though, this is another one of those bizarre "My minivan has a top speed of 110 mph but yours has a limiter that kicks in at 95 mph, so neener neener" types of things. You can already make absolutely stunning 24" x 36" prints from any of these full-frame cameras with ease, and you can max out the width of a 44" printer with a single exposure with care so long as viewers won't actually physically be sticking their noses in them. And for what those kinds of prints sell for, if you really need to deliver them with any type of regularity, you can easily afford the medium format gear that's actually designed to do that sort of thing.
I'm not at all exaggerating when I state that a 5DIII with a TS-E 24 has significantly more image quality than anything Ansel Adams ever laid hands on, including his 8" x 10" view camera.
So, while more is awesome, can't we be happy with what we've already got?