What he says about it is flat wrong, so I am sceptical of his observations.
He says that the lenses have to go beyond marked infinity focus to focus at infinity, and some do not reach infinity focus. This, he says, is because the flange focal distance is too short: "[...] the flange focal distance on the D850 looks to be in gross error. I estimate that it is off by at least 100 microns (0.1mm), i.e., the flange focal distance might be 46.4mm instead of the required 46.5mm."
But a lens is at infinity focus when the distance from the rear principal plane to the sensor is at its shortest (and equal to the focal length). The lens focuses closer by moving the rear principal plane away from the sensor. That is why you use an extension tube to reduce closest focus distance, and why that means you lose infinity focus. So if the flange focal distance was too short the effect would be increased closest focus distance, and infinity focus at closer indicated distance than infinity. What he is describing would imply a flange focal distance that was too long.
Even if you allow for getting it the wrong way around, he is wrong. The effect of a 0.1mm difference in flange focal distance on infinity focus is minute - that is why we don't use really small extension tubes. It is possible to do a rough calculation using the lens equation: 1/f = 1/u + 1/v, where f is focal length, u is distance to the subject and v is distance to the sensor. If we take f to be 50mm, and take the rear principal plane to be at the rear flange, the difference between v being 46.4mm, 46.5mm and 46.6mm is focus at 645.2m, 666.7m, and 684.9m respectively. The rotation of the focus ring is inversely proportional to the subject distance, so for a lens with 70mm diameter focus ring and a quarter turn from closest to infinity focus that 20m difference corresponds to a movement of the focus ring of two microns. I don't think you can detect that.
