Zane did train quite light weights when I saw him. They didn't call him the chemist for nothing. I think the pros that we worshipped were great responders to steroids, they didn't know anymore about training than you or I.
Full body workouts using a PHA system with compound movements 3X per week is all that's needed to stimulate one's muscles to their natural maximum.
Interesting that you mentioned Zane and lighter weights. I heard this as well. As the story goes, in the mid '70s or so, a couple of years prior to his first Olympia win, Joe Weider took Zane aside and told him that, although he had a good physique and excellent symmetry, he simply lacked the size to be a serious contender for the Olympia. And so Weider supposedly told Zane to train with heavier weights and the rest is history. Since we now know that Weider, himself, knew less about bodybuilding than he did about marketing and self-promotion, my guess is he just put Zane on "better" drugs.
I've never even heard of PHA training until I read your post. Although I first cut back on volume in 2005, my real reduction began in 2012, when I started doing 3x/week full body training. Eventually, I reduced it to 2x/week and saw no difference. A couple of years ago, I read Doug McGuff's Body By Science, in which he advocates once a week workouts. I bought into the theory, but I couldn't buy into the practice. I eventually tried 3 workouts every 2 weeks and then, for a couple of months last summer, only once a week, before returning to twice a week. I had no reduction in strength or size as a result of the reduced frequency. The reason I went back to twice a week was because I liked going to the gym more often.
But after rereading the book a year later, along with the last book by Mike Mentzer, I decided to give once a week workouts a try again, and have been doing so since June of this year. My current once weekly workouts are somewhat longer than what McGuff and Mentzer proposed, but the lifting portion takes no longer than 45 minutes. However, I also do HIIT at the end of the routine in addition to either one or two more HIIT workouts on the weekend. But they are very brief (and, of course, intense). I can tell you that I have not noticed any change in body composition, with arm, chest and leg measurements remaining essentially unchanged. Only recently, in the last few weeks, I have gotten slightly weaker on pull ups and dips (both weighted), but this is because my left shoulder is acting up again.
I won't pretend that I have the same physique I had when I was in my 30s, or even my 40s, but there has been no drop-off, in either size or strength, that I can directly attribute to the substantial reduction in volume and frequency.