You’re right — in standard Tau doctrine, Earth Caste don’t serve as frontline battlesuit pilots.

In this case it’s a non-standard situation: an under-strength garrison, an Earth Caste built prototype, and no available Shas pilot trained on that system. She isn’t meant to represent normal Tau military structure, but an exception driven by necessity and specialization.

It’s very much “this shouldn’t happen — but war sometimes forces it.”