Now I can't remember where I read it in-game (and it's not like I can go hunting right now), but iirc Gridania is only about 500 years old.

((This also forms part of our "the grand companies existed at the last calamity" conundrum.))

The wall markings inside the various Gelmorran ruins we've seen so far suggest the concept of the twin adder, representing unity between Elezen and Hyur, existed before the emergence into Gridania.

On a tangent, would it be reasonble to doubt that the padjal are the only keepers of white magic secrets? Could the Hyur have brought it back in their migrationary waves after it was banaished in a preceding umbral? Could White Magic have laid the foundation for the Conjury that resulted in reconciliation with the Elementals?