I don’t disagree with that concept, it does make sense and would account for some differences between doing a simultaneous adductor stretch (abduction of both legs) in standard splits compared to alternating adductor stretches (abduction of one leg at a time).
It’s also easy enough to see when you try doing a lunge/frontsplit type facing up stairs as opposed to flat on the ground.
Rather I’m skeptical of the argument that lifting your foot onto a hip-height stool a leg’s length away is only accomplished via 90 degrees of a single leg’s hip abduction. That doing this test is an easy way to know you’re capable of 90+90=180 side split and that there won’t be joint socket or ligament problems when approaching those extremes.
You can do that with 45+45 abduction too, via pelvic tilt. Instead of just the free leg’s abductors pulling the knee higher, change of angle can also be cause by the base leg’s adductors tilting the pelvis resulting in the free leg’s hip joint elevating.
You can TRY to keep your pelvis level but it’d be pretty hard to guarantee you weren’t compensating slightly.
Even if it was something like 10+80 because you have good body awareness and minimize compensation via lateral pelvic tilt in this test, 80+80 is only a total of 160 degrees which means you’re proving 180.
Even as much control as 5+85 only proves 170 which is short of the desired ‘roadkill’ relaxation that Pavel aims us at.