^Why not? Superman was made to look pretty bad in MoS.
What I read said that they are using The Dark Knight Returns
as inspiration, which is troubling, because TDKR's version of Superman was very unflattering to the character, and its version of Batman was meant to be an extreme, dystopian exaggeration. It's totally the wrong work to look to as inspiration for the first Superman-Batman meeting; it's the story of two old allies who've become rivals and come to resent what they see as each other's betrayal.