This explanation sounds entirely plausible. Most 5DII users were not screaming for better sensor performance - they wanted better AF
At the same time, this real sensor upgrade on the 1dx explains why Canon was so lenient to put the 1dx's af system in a much "cheaper" camera body, though at a lower af speed as far as I understand it. They felt sufficiently confident that the 1dx would blow the 5d3 out of the water anyway - not only for better built and more shutter cycles and so on, but simply due to the 1dx's high-iso shots making a real difference unlike 5d2/5d3.
Well, I think they needed to do something big for the 5d3's AF system on top of whatever else they were doing because so many people seemed that was a big failing of the 5d2. If they hadn't done something that big, they might not have gotten all that many people buying the 5d3.
That, and they can also harness the power of mass production to make much of the AF system for both 5d3 & 1DX cheaper on a per unit cost since they share large parts. Same with the firmware, they probably could take large parts of the firmware from the 1DX and put them into the 5d3 for the AF system.