Well, naturalism doesn't mean actually being like reality, it means creating the feel
of reality. Donner's buzzword in the production of Superman
was "verisimilitude" -- a word that literally means "similarity to the truth." I.e. not actually being the truth, but resembling it, looking and feeling like it. Yes, the characters acted in broad ways and did physically impossible things, but Metropolis had the feel and texture of a real city, the Planet
felt like a believable newsroom rather than a Hollywood set, and when Superman took off, it looked like he was actually physically rising into the air. Not to mention that Christopher Reeve acted like he was really, matter-of-factly Superman instead of giving a campy or self-conscious interpretation of Superman. So yes, it was a naturalistic take on a superhero film -- certainly a damn sight more than Burton's or Schumacher's Batman films, say.