Garlean attacks aren't magic, so what are they?

[Absimiliard]

Artificial magic-like effects created using ceruleum as fuel. Pretty much everything they do is technological in origin.

Oh, and you're correct about them not being able to manipulate aether. There are some short-term workarounds, like embedding crystals in their own flesh, but pretty much every avenue they could come up with that didn't involve magitek is excessively dangerous for them to attempt. Even the Resonant procedure poses an incredible danger to them, not that they even have access to the information on it anymore. There's a line in one of the Encyclopedia Eorzeas implying a small number of them might have some capability with aether, but it isn't really touched on anywhere else. The closest we get to Garleans directly wielding aether is RPR, but it too requires an intermediary and comes with a hefty price.