> sex and gender were always seen as one in the same and interchangeable

Various cultures always have had different norms and rules for the sexes. What might appear manly manly in one culture might seem effeminate in another culture. Just see how makeup was considered manly in ancient Persia and how it was ridiculed as woman-ish in Greece.

There have always been different interpretations and manifestations of what it means "to be a man" or "to be a woman" - that's gender. Some cultures - like some native American people - recognize expressions of sexual identity disconnected from their biological sex.

So no; sex and gender were not "always seen as one in the same and interchangeable".

I agree that sex/gender are irrelevant and that every human has the same rights as every other. Including the right to not be discriminated against based on sex/gender.