In the prequel novel Riddler's Gambit set 5 months before Arkham Knight's events, Robin makes a slight reference to the superhero via a memory that he has.
Reference to Superman in Arkham Knight
"You think the Bat is tough to kill? You don’t want to go up against the other guy in the cape."
"That'll teach him (Batman). Who the hell does he think he is anyway, Superman?"
"You know the only thing that's missing is if that freak from Metropolis flew in here."
Reference to Superman in Arkham Knight: The Riddler's Gambit
'They reminded him of some robots he’d seen in an old Superman cartoon.'