It's pretty well established in dialogue and in a couple of scenes that Superman has been acting as Superman over the past couple of years.
Sure, it was asserted as a fact, but that's a far cry from actually being the focus of the story and characterization. That's one of Snyder's faults as a filmmaker -- he just puts things in because they're obligatory rather than actually having them arise organically within the narrative or have any emotional grounding for the characters or the audience. (Like how there was no story reason whatsoever for the World Engine to land in Metropolis and no effort was made to establish any connection between Clark and Metropolis before the climax happened.) One of the most common phrases I hear about his films is that things are not "earned" -- they're just tossed in. Snyder is good at making pretty pictures, but lousy at weaving them together into a healthy narrative. He's like those comic-book artists who think that being artists qualifies them to be writers, but whose solo comics are incoherent messes that egregiously mishandle the characters.