[Spoiler] A Garlean Pureblood using Aether?

[CyrilLucifer]

We don't have any evidence of Garleans "acclimatizing" to the use of aether; if it's possible, then why wouldn't the prolonged use of Ceruleum influence them? It's not like Garlemald is completely devoid of aether.

That's very hypothetical and I'm not sure I agree with it on the basis that such a possibility would very likely have become public knowledge in the past if it was possible.

My best guess is that it's a mistake and the lore team did not communicate with the battle/quest design team. It happens. I really, really wouldn't look more into it than that unless you're hoping for disappointment.