Yes, mechitza is binding, because it is a minhag nitpashet, just like kippah.
Yet nobody would claim kippah is a d’oraita (except the Shach who classifies it as part of lo selechu), and the Gra says there’s no hiyuv at all in kippah.
When it is claimed that a minhag hanitpashet is a d’oraita, that is highly troubling and disturbing to me. Although this radical thinking was probably undertaken as an eis la’asos since it seems uncharacteristic of Reb Moshe’s thinking in general