I'm on both sides of the fence here. One one hand I feel that high volume is good and conducive to gains. By using both 1x and 2x a week protocol.
I also feel that using a lower amount 1x & 2x per week is just as effective as doing higher volume.
What, and where, I feel people go wrong is by making blanket statements. Especially in regards to the human body you cannot just say X program is better than Y program. There are so many factors and/or parameters that go into why something may or may not work. One guy may have a barrel-like chest, so he's not going to get as much out of benching as another guy. Yet the guy that does a lot of benching may just not like using DB's whereas the former NEEDS DB's to achieve equal or lesser growth.
When it comes to volume, per overall amount of sets or per body part, BOTH work. Neither work.
If you're doing 9 sets per part, you'll bump growth up if you jump to 12 sets per part for a time. You can even ramp it up to say 15 sets. Then when you cut back again to 10 sets or a lesser amount of volume, you'll grow again. But where high volume trainers go wrong is that they'll preach to someone that's already (and has been) doing 18 sets per part, so that person bumps it up and they see no growth. Where the low volume/HIT'ers tend to go wrong is that they'll advocate a high volume guy cut his amount, but the high volume guy either can't put all his effort into the lesser amount of sets, or he just doesn't stick with it for a good duration to see any of the gains. He's mentally used to grinding out so many sets and that's where he can get psychologically trained to feel a certain way.
ALL programs work. Nearly every single program out there has been backed up by science. Pick out 10-20 "champs" from all walks of life over the last 20yrs, and you'll also have evidence to support the all programs work credo. Yates used a variation of HIT. Metzner was similar, but wanted even less sets, but is rumored to have done more than what he espoused. Ronnie Coleman and Arnold used tremendously high volume 2x a week to achieve their gains. You have supposed "naturals" using both programs and getting gains from it- far from their enhanced lifting cohorts though.
You can make a case for anything, as long as you know how to spin the numbers to yield what results you want.