He's still Kryptonian but the repeated processes used by Apokolips have damaged his cells to the point his ability to store and process sunlight is all but gone. Bear in mind he has been hidden deep beneath the Firepit for over a full five years now, his powers would have faded and made him all the more vulnerable and unable to recover from his constant abuse. Considering this It is a wonder he is as healthy as he is...
Superman's fate is interesting in that this appears to be the same fate intended for Earth-1 Superman in Geoff Johns' early Justice League - Apokolips clearly has a great interest in Superman and his power.
Is Superman somehow tied into the new gods in some uniaque fashion, and just why does Seid seem to have almost a fetish to conquer and defeat Superman all the time?
Why has he not focused on getting E 1 kal beaten down and controlled so far then?