I would not compromise thoracic extension to increase range of motion in the hips. You start to round the back so the range of movement is spinal flexion and not hip, knee and ankle flexion. The hips tuck in at the bottom creating lumbar flexion. Debate regarding allowing the lumbar spine to flex is huge amoung coaches and spinal scientists. If the lumbar spine flexes from its fully extended position but does not pass a 'flat' back position and does not go into flexed position, I believe it is a safe position to squat.

These are points that are not clearly highlighted in the video but are important for spinal health and perforamce. Reading and listening to Stuart McGill, a Professor in Spinal Biomechanics, the closer an athlete lifts to world record loads in squat and deadlift, the better the athletes spinal mechanics are - no thoracic flexion, no lumbar flexion and no hips tucking under the spine. An athlete will make limited progress while exposing themselves to spinal risks if these issues are not addressed.