When you extend your hip as in a hip flexor stretch and flex your knee, as in Ian’s version of the stretch, this will cause your rectus femoris (a quadriceps muscle) to be stretched since it crosses over the hip joint as well as the knee joint.
If you feel you’re a only getting a stretch in your quads this means you have inflexible quads (specifically the rectus femoris).
For you to adequately stretch your hip flexors right now you should just perform an ordinary kneeling hip flexor stretch, which is just like the one Ian describes, but not placing your back foot up on a bench.
On top of this you should also work on your quad flexibility since your rectus femoris is already inflexible. You can use any quad stretch that you are comfortable with right now; however, conciously try to extend your hip back which will stretch your rectus femoris more.