But even if they were right and it's all just a terrible mistake, all just social conditioning based on arbitrary assumptions, so what?
Does that mean any disadvantages that happen as a result of that conditioning magically didn't actually happen? That all the data that show people who meet that boring old sex-based definition of women face risks, challenges and headwinds that the people who people who meet that boring old sex-based definition of men don't are somehow no longer true because the reason they were disadvantaged was based on a false premise? Of course not.
Again, words are being twisted to hide the truth. Changing which people count as "women" doesn't change which people are female, or stop society treating us like it always has.
It's telling that the reddit poster picked two privileged and stereotypically white nations as the reductio ad absurdum comparator for socially constructed differences.
Yes, Belgians and Australians might be a laughable comparison, but no one would suggest that the difference in experience and social challenges faced by Black British and White British people (or Black Belgian and White Belgian, or Black Australian and White Australian) is trivial or laughable. Yet race differences are absolutely 100% arbitrary social conditioning.
Would the men (and for different reasons, the women) who are so amused/enraged by the concept that the social consequences of being female can be significant and not mitigated simply by one declaring one does not want to be constrained by society's beliefs about ones sex, and so confident in their opinion and so ready to share it, be as cavalier if it were the consequences of race that were being dismissed? I don't think so.
When it comes to race, anti-racists understand that it's perfectly possible, indeed essential, to aspire to and actively work towards a future where race doesn't matter to someone's opportunities but also recognise we are not there yet so people will for now still experience racism and need still support.
Yet when it comes to female people, suddenly the narrative becomes "this shouldn't happen, so admitting that it does happen and supporting the people who are impacted is as bad as saying it's ok".