Further to what Doug stated, it's always been possible to 'trick' Canon's f/5.6 AF points into focusing with an f/8 max aperture, with pin taping or a TC that doesn't properly report. Canon did not allow the functionality because it wasn't completely reliable. Their recent firmware, possible coupled with higher density line sensors on the 1D X/5DIII (which is how the past 1-series bodies achieved f/8 AF), allowed them to enable it with confidence.
I also suspect they built the capability into the 1D X/5DIII sensor, but left it disabled in firmware until later.
OK, and now for my next dumb assumption ... at f/11 the focus needs to be less accurate than at f/2.8 due to the greater depth of field (front & back) of the former over the latter.
Correct. The smaller the baseline of the sensor, the less accurate. But, the narrower the selected/available the aperture, the deeper the DoF and the more inaccuracy tolerable in terms of focus.