I'm not talking about spare the spine, I'm talking about hip socket depth affecting squat depth. I've noticed there's not much difference in knee and back angle between my high bar and low bar squats.
Setting aside the issue of "is buttwink dangerous," keeping your low back straighter seems to help engage the hamstrings as a hip extensor out of the hole.
The difference between a low bar and high bar back squat is about an inch....maybe 2. This isn't going to make a huge difference in back or spine angle.
The further you bend forward (horizontal back angle) the more your hamstrings are going to be engaged. For general strength training this is superior because of the additional muscle recruitment. There's a lot...maybe even most powerlifters that squat high bar because of this. It's a small difference but saving the hamstrings for the deadlift could make a difference there.
When you look at olympic lifts, especially the snatch they keep their back/spine angle artificially vertical because the pull from the floor isn't the hard part of the lift so they aren't looking for max leverage there. They want the hips low and under the bar for the thrust and jump.
I don't think McGill is very good at coaching the squat. Start the squat with feet shoulder width apart and with the toes somewhere between 33 and 45 degrees out. Adjust for comfort and depth. Guys with a boiler are going to need a wider stance. I sometimes use a wider stance for comfort when I have a weird act or pain. The answer for the guy in the red shirt is to admonish the lifter to push his hips back and get into the hamstrings. It just takes coaching and time. To try and tie in to hip socket depth or anything else is just for clicks and views. What feels natural tis usually the right width.
I submit this for evidence of silly bullshit.