I'm not sure our usage of gender (or sex) is clear enough for everyone to be onboard with that distinction between gender and sex.

Things that are a matter of culture ("social construct") are clearly gender.

Things to do with the biology of reproductive organs is clearly sex.

The biology of differential brain development that may lead to innate behaviors that matches what is appropriate for one's reproductive organs is...? Um?

Even if you don't think this happens, not everyone agrees that this doesn't happen, so you still have to account for the possibility, and that possibility is something to do with...? Sex? Gender? Both? Neither?

In the usual case where everything aligns, you don't really have to worry. In rare cases where there's some conflict, it starts to matter. And this is language intended to handle rare cases. You make a good case that what they chose does so poorly.

But I think you're overstating how obvious it is that the original language was completely unambiguous and uncontestable. In particular, I don't think it's entirely clear what if anything falls under the "sex" label beyond non-brain body morphology that supports reproduction.