Most of the movies before this were based off of stand alone stories themselves. There really was no reason to reference any of the others. Does Superman exist in the Under the Red Hood universe? We don't know. There's no reference to him. I assume he does because of Amazo but there's nothing in the movie itself that states this. Now, Superman obviously needs to be part of Hush because that was in the comic. But it doesn't have to be a specific Superman. The Superman/Batman movies appeared to be part of the shared universe but it wasn't necessarily part of anything else. The movies based off of Miller's books don't necessarily need to be part of the same universe but they can be. And the other Justice League movies are up for debate. Doom. Crisis on Two Worlds, etc.