I'm guessing here, but I think when he claims that focus has no bearing on image quality he is referring to the parameters that are not under user control. User can not remove noise or add DR to a sensor, but a user can change the focus. Just a guess.
The user CAN remove noise (lower ISO, ETTR, processing, stacking) and add DR (lower ISO, ETTR, stacking, HDR).
I guess all of that applies when the user is taking a picture of a galloping unicorn on a rainbow. Pitty that most of the real world situation are not as easily corrected.