This is not true.The two terms were used synonymously for decades if not longer. Its only when linguistic sophists decided to codify and normalize a mental condition on the back of the gay rights movement did those terms get set apart.
In the Oxford English Dictionary, gender is defined as, "n mod[ern] (esp[ecially] feminist) use, a euphemism for the sex of a human being, often intended to emphasize the social and cultural, as opposed to the biological, distinctions between the sexes.", with the earliest example cited being from 1963.
Sex and gender distinction - Wikipedia
Gender has meant different things in different cultures at different times. It's never been as cut and dry as that.
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