I think we all agree that it’s against halacha to dress not b’tznius.
I think we also all agree that it’s not always easy to clearly deliniate what is considered b’tznius.
So we can’t figure out the actual halacha by looking it up in Shulchan Aruch as I think (or at least hope) we can all agree that there are clear violations of tznius which are not found explicitly in Shulchan Aruch.
So we leave it to the poskim to determine whether individual cases of styles of dress conform to acceptable tznius standards.
Rav Falk zt”l was a big talmid chochom (just read his teshuvos on a broad range of topics) and was certainly entitled to his opinion as to what does and does not conform to halacha.
Someone claiming that what he said as halacha is really chumra better be in his league in learning to have such an opinion. If your rav says he disagrees, fine, but just realize that Rav Falk was a massive talmid chochom and he held something was assur (not chumra) then it’s not proper to dismiss it as chumra without qualifying that “my rav disagrees and says it’s not absolutely necessary”.