It's not about the lack of pixels but the space it affords them.
The pixels are what? 10% bigger than the 5D3's?
ISO stops = 100,200,400,800,1600,3200,6400,12800,25600,51200 - 10 stops.
10% bigger pixels = extra stop of performance.
This is irresponsibly terrible logic. You found the number 10 twice (incorrectly, and in arbitrary places, I might add) and concluded that there's a relationship. I'm not sure if you're parodying terrible logic or are serious; my apologies if your post was a joke (subtle humor is lost on me over the internet!).
Assuming gapless microlenses, the 1DX has pixels that are 23% larger by surface area (5760/5184)^2, not 10%. And it doesn't matter because the surface area that collects light is roughly the same on both cameras. If the 5D has smaller pixels, it has more of them, and this cancels out almost exactly except maybe in terms of collecting light at very oblique angles. The D800 is maybe 3-4% less light efficient than the D4 despite having more than twice as many pixels--that's the tiny extent to which megapixel count effects light sensitivity (assuming you have a very clean ADC, as those cameras do).
The issue is likely with read noise. The 5D III (if I remember correctly) is >50% efficient, an improvement over the mark II that is achieved with gapless microlenses, but it has very high read noise. There 1DX likely has very similar quantum efficiency but a better ADC. It obviously has a better sensor but not because there are fewer pixels.