I made a bad business decision once and mandated that my training company use the OPT model exclusively. We all went to the CPT workshop where the instructor stated "the reality is, most of your clients will never make it out of IST." How right she was. Therefore, the majority of our clients made VIRTUALLY ZERO progress toward losing fat and increasing LBM.
Even with NASM-like flexibility, it is virtually impossible to squat (at least FULL squat) like an accordian as they suggest. The only type of squat that comes close is a front squat, and sometimes an overhead squat. Forget it on the back squat. The feet need to be just outside shoulder width to allow the hips to pass through the legs. As far as this putting the glutes in a shortened position, this is only true if the feet are externally rotated. If the feet point straight ahead in a full squat, as you ascend, the knees will pull in toward the center of the body (naturally) and actually STRETCH the glutes to store and then use elastic energy from the stretch to aid in keeping the torso upright. But that's a whole 'nother discussion.
Hope that encourages you.