The question wasn't whether or not Superman could win, it was whether it was better for the public for him to use any means necessary to stop crime. The fight is only demonstrating to the public that a Superman who uses whatever methods he feels like is actually terrifying.
I don't think Superman is trying to convince the elites in that scene; he knows that they're just jerks pretending to be heroes. That's why he ends up permanently disabling Manchester Black's powers. He cares more about the public knowing why he doesn't kill villains for them.