Yitay,
It is not uncommon for halachos to be derived from diyukim in Shas. It is clear from the Mishna and the Gemara that there is a concept of a minhag of B’nos Yisroel in tznius above and beyond that of the general population (it’s called Daas Yehudis, not Daas Nashim).
That such minhagim are binding is clear from the Gemara in Pesachim I cited, and many halachos in Shulchan Aruch that have their origins in minhag (like wearing a head covering for men).
I don’t know where you live, but where I live the frum women still cover their arms and certainly their bodies and legs. Such customs are therefore binding on all Jewish women in the community.