Eorzea has much Light? How?

[Iscah]

So, they both were part of an original Mothercrystal, and while we now equate Hydaelyn and Mothercrystal, it's only because she (technically speaking) usurped full control from Zodiark, who was split off into a moon. This entire section is sourced from the Scions themselves, though, so, ahem, "new revelations" could always change this interpretation down the line.

It depends how you interpret that explanation - I was literally just about to post that I'd checked the lorebook and it didn't support them originally being one entity!

More exactly, it seems that they may have physically existed together within the Mothercrystal, but as separate "wills" of Light and Darkness within it. Thus the description referring to Zodiark being "physically excised from the planet", cast out from Hydaelyn-the-planet, not split from being a single entity sharing the mind of Hydaelyn-the-goddess.

It seems that their not being one and the same was the reason for Zodiark being banished in the first place.


(But yes, everything comes with the footnote "as far as we know at the present time".)