hey my AT teacher never said that we are too weak, I agree that's an awful way to put it,
the principle is all start from the neck, if the neck muscles are relaxed your shoulder will be relaxed, the vernal/muscular damaged is caused by both excess of tension and posture, and ironically if your posture isn't perfect but you learned to keep your neck and upper torso relaxed, you are very unlikely to get any pain.
Conversely, perfect posture but lack of control in muscle tension will eventually lead to pain and problems.
Unfortunately, it isnt something can be told or explained with words, its more absorbing the neutral posture via teacher guidance until it overwrite the old bad habit.
Think about keeping your head pointing forward and up, without engaging muscle its more about balancing the head over the neck.
that will consecutively relax the upper back and shoulder muscles, always start form the head, neck and down.
you can turn your head a little to look at the fretboard (with a soft focus!) but always needs to stay relaxed and balanced.
a few AT lessons aren't enough, I've started changing posture, naturally, after 20 something lessons.
It feels like none of that makes sense at first but it's quite amazing when the transformation occur.