What exactly is going on with the Xaela's civilization?

[Lurina]

I don't think the origin story for Azim and Nhaama was quite close enough to Hydaelyn and Zodiark to say it must be preserving that exact tale. I would think it more likely that they were visited by an Azem at some point (be it Venat, our past self or some unknown predecessor) and something happened that was significant enough to become part of their mythology.

I dunno about that. It's a war between a light and dark aspected deity (of which the dark one is associated with the moon) which involved the creation of the first mortals. While it's obviously corrupted, that seems like Hydaelyn vs. Zodiark to me.

Plus, the Au Ra didn't even exist until the Sundering, like all the other modern races.