“A pasuk is never used to counter a Rabbinic enaction. However Rishonim who we don’t pasken like, are often used to rely on (sometimes with being metztaref other factors) in a shaas hadchak. Hence they have halachic weight whereas your pasuk doesn’t.
“How can citing a source from 800 years ago have any bearing on a minhag that has developed since then?”
Because the minhag that developed might have been a minhag shtus. Now if you want to quote other Rishonim/Acharonim who disagree and hold that it’s a good minhag then fine, but that’s a different argument then the one we are discussing.