The point is that why should a person receive more comfortable and lenient treatment in prison simply on the basis of their chromosomes?
The answer isn't because of their chromosomes but that as the sex class women, they have been oppressed by the sex class men. No amount of identifying wearing dresses or anything else nullifies that fundamental experience of being a woman. That is why women should be in single sex prisons (and else where) because the oppressing class, men, are not just triggering but the source of potential danger.
It is as ridiculous as suggestting that because a white person indentifies as black, somehow they lose all the race based priviledges they have experienced throughout live.
The oppressing group, men, however they dress or identify, are still the sex class that oppress the sex class of women.
(And by the way the issue wasn't about women being able to wear dresses, but the fact they are according to the post allowed to wear their own clothes - which might well be jeans or leggings - but male prisons aren't. You need to start a campaign to give male prisoners the same rights in terms of clothes as women if they is what worries you.)