Not true; it is required for devarim she be kedushah.
Actually, the requirement for the mechitzah is only with both conditions. Technically, a mechitza is not required for a minyan at a chasunah, for example.
And as mentioned it is really hard to say that it is more than a d’rabbanan since there wasn’t one in the beit hamikdash until late in Bayit Sheni times
That’s circular; if you hold it’s d’Oraiso, you assume it was there during the first bayis as well.