I would tend to agree with you on this point. And even more could it be suggested that an increase in hip felxion can often be achieved with a corresponding reduction in hip extension?

We certainly see that excessive external hip joint rotation can be coupled with a reduction in internal rotation, yet overall movement might remain rather constant.

It might be worth while to discuss what one is actually trying to achieve in measuring hip flexion. Perhaps it is not the best measure for the question being asked?