Adam's take on Quick is quite accurate. I've spent time with him and with Kim
Brackin who coached both the Olympic and World champions in Women's 200 Back at Auburn and subsequently at Texas. She does not teach anyone to arch the back, but to maintain a balanced and streamlined position in which the pelvis is relatively neutral, the spine is straight and hips touch the surface alternately.
The first three drills of Backstroke for Every Body
teach those positions.