Yes but we know precisely what the Maidens are too (Gods, created by the Primordials). They just weren't created in this timeline.
Which doesn't really take into account that Gaia has more than one body. You'd think the Primordials, of all people, would remember that they can be in more than one place at once.
Also, regarding hero styles: they are not martial arts for the Exalt types they represent. They're just Martial Arts (or Dexterity) charms. They're formalised martial arts designed to imitate those exalt types when someone else learns them, but Lunar Hero Style is just how Lunars fight.
So yeah, actually, there is a garuntee that other people can learn them.