I think the idea is that you want to keep as normal a posture as possible. You need to lean your torso forward to move the combined center of gravity (body+backpack) over your feet.
The lower the center of gravity of the backpack, the more you need to lean your torso forward to counteract it.
If the center of gravity is very high, just a few degrees forward will suffice.
So a high center of gravity should be more comfortable (but you lose a bit of stability).
It also makes a huge difference how close to that 33% body weight max you carry. What kind of backpack carrying people do affects what kind of advice they've heard.
The weight of a military pack will absolutely send you toppling over if you lose your balance, and you need to start packing it a bit lower just to deal with uneven terrain. So you pack it a bit lower than what would be otherwise ideal, to stay on your feet.
If you're carrying closer to 10-15% of your body weight, you can put it much higher without stumbling, and you get the benefits of the more upright posture.
I understand the "close to the back bit", but "above shoulder height"? I've always been told it should be distributed vertically.