I did my calculation as well, but I think canon also used other techniques (something difficuilt for consumers to find out) to achieve this. I don't think canon is that stupid to let us discover this just by any simple calculation.According to Canon USA the image format of 6D is 35.8mm x 23.9mm, 5D2 is 36mm x 24mm.
I guess Canon just trim down the 5D2's sensor a bit. Maybe the AF is also modified from the 9 point AF in the 5D2. In this way, Canon is able to use these 4 years old parts to create a new body, reduce the overstocked 5D2 sensors and AF module and cut the cost a lot. Three birds with one stone.
Nope. I did some quick calculations and the number of total pixels on this sensor is less than the number of effective pixels on a trimmed-down 5d2 sensor, so the pixel pitch is definitely different.
Besides, modifying a stock of existing components is, even if it's possible (and I really doubt that it is in this case), hardly cost-efficient. It's just plain easier to produce a new piece from scratch.
All of these are just my speculation though.