Garlean's can't normally use aether, but Zenos has been since he became a resonant, even before his temporary body displacement at the end of Stormblood. We don't know why Garleans can't normally use aether but maybe having the echo/resonant can overcome whatever reason. Emet-Selch could use aether as well even when he was in a cloned garlean body on the Source.

I did wonder as well if the "feeding on souls" bit was literal, but I jumped to "is he trying to become a primal?" not "reaper". Considering that reaper is going to be a player job if he is feeding on souls because of his job its probably an aberration and not how reaper normally functions (like Lambard in the rdm quests).