First of all he is incorrect in that the movement is a stretch of the external hip rotators and abductors. It can stretch the sciatic nerve when the bent knee is away from the torso. Bringing the bent knee closer to the torso stretches above the sciatic notch. When the pelvis is allowed to move such that one ischial tuberosity (sitz bone) is forward of the other then the sacral ligaments and sciatic nerve can indeed be in a compromised position. In the Sept/Oct 2001article on the sacrum Judith Lasater addressed this concern quite thoroughly.
I have not seen Dr. Stark’s book. However, it sounds to me that Dr. Stark was evaluating some methods of stretching in which anatomical and kinesiological knowledge were not taken into concern for body placement. Perhaps he is evaluating a different movement than what we know as a well presented Yoga pose, I cannot be certain. Good Yoga practice will be based on sound anatomy and kinesiology knowledge from which one may need to adapt to individual’s anatomy. Yogis are learning more about this every day. I hope to be able to contribute to that knowledge and dispel misconceptions. After all we are all built differently. And there are plenty of places where body position will create more problems than they alleviate. You have brought some good questions. Thank you for calling our attention to these concerns.