i don't think anyone has said the 6d af is in the same ballpark as the 5diii. Center point only, its probably a tie, but everything else its not even close.
I have both the 6D & 5DIII, and the center point only focus on the 6D is better, particularly in low light. The 5DIII will just stop working if the light get's too low, but the 6D remains accurate. In fact, in my use that center point on the 6D is the best AF sensor Canon has ever made. I have had a higher 'hit' rate of in focus photos with that camera than I have with any other, and that includes the 5DIII. Obviously, the AF isn't as versatile as the 5DIII, it wouldn't be very good for shooting butterflies, for instance. But for portrait work, and slow moving subjects, it does a fine job. I don't notice it particularly slow in focus either, I have used it at several weddings and have yet to find a situation where I felt I missed a shot because the camera wasn't fast enough to lock focus. In very dark situations, I have had that happen with the 5DIII, so in those situation I actually prefer the 6D AF, whether the subject is moving or not.