What do you mean? He was that universe version of doomsday. They said so. He killed superman and everything. It was done poorly
Luthor called him "your Doomsday" that was all. Snyder also confirmed that the real Doomsday was out there in the DCEU. So this was just a movie villain not a comic book villain. I didn't like it personally they should have chosen something completely new that was issue with BVS along with a version of Luthor i didn't like.