It would seem to me that the AF module and the FPS are mixed up between the two really.
The 1Dx churns out 18Mp x 12fps = 216 Mp/s
When you reverse it on a 45MP sensor, you get 4.8fps.
So, it doesn't make much sense to get only 4.9fps on the 5DIII if it "only" has 26.4MP. Likewise, it doesn't make sense to put a top of the line AF on a slow FPS camera either. When in a sense, a 19pt 7D sensor is quite suffice??
I would think, that the specs should read :
5DIII: 45MP, 19pt AF, 4.9fps, High Dynamic Range, moderate ISO 12.8k-25.6k.
It would make sense as a high pixel camera for landscape, studio, product work.
5Dx : 26.4mp, 61pt AF , 8 fps High ISO 51.2k- 102.4k,
It would fit into the role of a compact, photojournalist, sub-sports, do everything camera body.
Makes more sense