If you want to measure if a woman chooses to graduate in STEM or some other field, you have to measure the women who got that choice and not the women who weren't given that choice. Their example "reversing" the result changed it to also include all the women who can't go to college at all due to gender discrimination and thus didn't get a choice on the matter, that doesn't help us see whether those women would have gone into STEM or not at college.

That is why you have to compare women who choose STEM to women who choose to study something else. An anti equal society would be expected to push women into feminine fields and away from masculine fields. But as the study showed that didn't happen, gender unequal societies seemed to do the opposite. All this counter study showed is that you can mask this bias by also weighing in that unequal societies also bars many women from entering college at all.