First off, McGuff and Little cherry picked their research. Second, McGuff has been a long, long time advocate of SuperSlow training, yet hasn't managed to put on any appreciable muscle in 10+ years.
Oh, they make a very, very good case for this way of training. It's too bad that anecdotal evidence has never substantiated it, nor has the VAST majority of training experimentation done my legions of exercise physiologists, biomechanists, and sports scientists.
Ask yourself this:
What successful athletes have training using the Body By Science protocols? Not just used them at one point in their careers, but used those protocols to develop themselves into the successful athlete they are today?
Also, training frequency is just one variable in a training program. As is the "intensity of effort". When have you ever seen a complex system, like the human body, respond to only one variable out of 10s, or potentially 100s?