"Miounne: Even now, the wounds have barely begun to heal. Ah, but I speak of it as if you were there. Forgive me. Five years past, Eorzea was well-nigh laid to waste when a dread wyrm emerged from within the lesser moon, Dalamud, and rained fire upon the realm. It is this which people call “the Calamity.”
Miounne: Scarcely a square malm of the Twelveswood was spared the devastation. Yet despite the forest's extensive wounds, not a soul among us can recall precisely how it all happened.
Miounne: I am well aware of how improbable that must sound to an outsider... It is improbable. But it's also true. For reasons we can ill explain, the facts surrounding the Calamity are shrouded in mystery. There are as many versions of events as there are people willing to recount them.
Miounne: Yet amidst the hazy recollections and conflicting accounts, all agree on one thing: that Eorzea was saved from certain doom by a band of valiant adventurers.
Miounne: Whatever else we've misremembered, none of us have forgotten the heroes who risked life and limb for the sake of the realm. And yet...whenever we try to say their names, the words die upon our lips."

Maybe they just don't remember there ever being two moons?