It is not semantics. I was explaining R’Moshe not giving my own opinion. Kalus Rosh is an issur d’oraisa according to R’Moshe. Therefore one is required m’doraisa to take the necessary steps to prevent kalos rosh.
The decision to use a balcony or mechitza as the means to prevent kalos rosh is d’rabbanan, but something must be done m’doraisa. This explains why there was the balcony was taken down after Yom Tov and why they only implemented the balcony in Bayis Sheini.
This is also why R’Moshe requires separate seating at weddings. Not because there is a din of separate seating but because of kalos rosh.
This also might be why the Shulchan Aruch doesn’t bring down mechitza as a din. He does bring down Kalos Rosh as issur chamur and he emphatically states the efforts needed to distance oneself from woman (who are forbidden).