The lore book specifically mentions that aether-users in the Garlean legions are highly valued (no matter where they come from/what race they are) and that the Garleans haven't been able to use aether since before they were driven to Northern Ilsabard in the first place. In fact, them not being able to use aether is why they're in Northern Ilsabard at all. Centuries ago, the Garleans used to be much further south and have been driven steadily north by aether-using races who wanted their territories. Once they got to Norther Isabard (FFXIV Siberia?) there was nowhere else for them to go and they've constantly been at war with the surrounding people to keep from being overrun. It was only with the invention of the curulium engine and magictech that they were able to get on an even footing with the other countries at all never mind take them over.