I listened to almost all of that 3 part interview. He got some good points, but at the point when he said "anything more than 1 set is overtraining" and "99.9 % of athletes overtrain" I couldn't take him seriously anymore.
I thought about his use of the term "overtraining" and I think he uses the term wrongly. Overtraining is a state of chronic fatigue (unless the meaning changed over the years, which I doubt). It's ironic how Mentzer is trying to come off as a really scientific guy but doesn't know this simple definition.