It's introduction and parallel with sex means the article makes a wider point about a difference between sex and gender. I agree this article could have been written better without any mention of gender so as to avoid making an out of scope claim in a pop sci article.
It denies the different "gender roles"
> Does that not mean that the trans community is the strongest supporter of traditional gender roles?
Um yes? That's been a sticking point for quite a while.
Because men are much stronger physically due to genetical differences. Just check out top records in various sports.
(That page has a link to download the PDF if you're interested, but also here's a taster in the form of a book review at the time it was published: https://www.nytimes.com/1979/06/10/archives/male-and-female-....)
For starters why are single sex sports and single sex changing rooms so controversial?
Similarly if gender and sex are entirely distinct. Does that not mean that the trans community is the strongest supporter of traditional gender roles? Afterall what does it mean to change gender if differing social attitudes to gender ought be extinguished?