I think they meant to say Etheirys, Lurina.

To answer the question, it's very likely they only use the Nethergate to first get down to the surface, and then do what everybody else does...attune to Aetherytes. They only need the Nethergate for the original transfer down, but the second they attune to any Aetheryte, they no longer need it to go to and from Etheirys.