I think the answer sayingtalking about clark beenClark being more used to his powers more is trueright, and zod ectZod etc do have troublestrouble with them.
BUT the codex bit is totally wrong, as the scientist on zodsZod's ship says he don'tdoesn't need to be alive to Harvestharvest the codex from his body n the.
The way supermanSuperman can defeat them is thehaving more experience with his powers, coupled with having been on earthEarth longer, so his cells have bathed in the radiation of the sun longer solonger; he has become stronger through that.