Well, I attributed it to something like legend. Elves don't exist in reality but we have plenty of legends and fantasy stories about them and so we could call something an elf if it behaved much like something from those legends.

So while Ostaria may not have elves they could have legends of beings called elves that could be identified by certain traits, possibly legends of Opradans interacting with the world from long ago. We never really defined a history or language for any of these cultures so it could be possible that the word "elf" may derive from an ancient form of another word that these cultures had for Opradans.

I'm very good at creating lore, especially to explain something like this within the bounds of the existing lore XD