Interjection and Yitay,
The Mishna in Kesubos is discussing things women can do that will lose them their kesuba. It says that someone who “violates Daas Moshe and Daas Yehudis” loses her kesuba. The Mishna then gives examples of each.
Rashi explains that “Daas Yehudis” are the minhagim of B’nos Yisroel. The Meiri explains that these are minhagim that B’nos Yisroel keep out of tznius so that Jewish women should have higher standards of tznius then the general non-Jewish population.
There are many, many sources that minhagim of a community are binding on all members of the community. See, e.g., Pesachim 50a-b. The language of the Mishna about Daas Yehudis itself assumes some sort of binding power. The Mishna uses the word “oiver” a term used for the violation and she is punished for her violation by losing her right to her kesuba.
Uncovering arms is classified in the Gemara as Daas Yehudis. No hair covering at all is deemed a violation of Daas Moshe and the Gemara says it is Torah prohibition. Partial uncovering of the hair (i.e. only covering the top) is deemed a violation of Daas Yehudis.
“Yes, I know very well that there is such an assumption. I am challenging it.”
Why? The assumption seems fairly reasonable, why would you assume the opposite, that women are free to walk around in public in a manner that would require all men to look away?