Anonymous rumors claiming Rocksteading wanting a Superman game don't mean it's true.
In fact many developers at the studio said years ago that doing a Superman game was a daunting challenge for them. Like Dax Ginn said,
“I think Superman is a very difficult character. He’s an incredible character, but the thing about Batman is that he’s so vulnerable, like physically vulnerable. The fertile territory of games design is being able to provide the player with opportunities for power, but also letting them know that they’ve got something to lose,” Dax said. He continued his explanation stating, “With Batman, you feel very powerful when you’re surrounded by 25 thugs who’ve just got baseball bats, but give just one of them an assault rifle and it changes the entire dynamic of the fight.
But with a character like Superman, that’s not possible. So, I wouldn’t want to throw Batman away because he’s just such a brilliant character to work with, and the psychology of him is just very rich for gameplay.”
https://gotgame.com/2011/10/11/arkha...ted-for-games/
Many others said that in interviews that Superman was a very difficult and daunting challenge to do in games.
So I think it's clear that they wanted to do a Suicide Squad game.