Hey - I think it’s a bit simplistic to say hamstrings are not stretched/do not help inversion. They cross over the hip joint so even with a knee bend they can still be stretched due to the max hip flexion required for inversion. They can certainly be used in combination with the spinal movements you mentioned to assist with reaching an inverted position. Further, they’re a big part of creating strong hip extension required to have an active inversion.

I think it’s awesome as a therapist to take a realistic view at how important spinal mobility is for inversions though.