I appreciate you taking the time to do some tests of your own. This is what I wanted to read, other low light focusing tests.
It's going to be very difficult to notice the difference between the 5Diii and 6D for single focusing type tests, both are going to focus in low light in just over a second. Even when going for 30-50 rounds of focusing, the 6D was only marginally better in the central point. Set up a timer and focus back and forth between 2 high contrast targets 50 times and then you will start to see the differences in speed easier.
As a side note, most of the 5Diii's central, top to bottom, middle row, did very well in low light, where the 6D's other squares failed.
As a whole the 5Diii's focusing systems are the best of the bunch (not even close), but when it comes to low light, yes, it does seem the 6D has the edge in that one central square. Its a true pity Canon didn't have more of an upgrade, but the 6D absolute dominated the 5Dii and D600 on that central point in low light.