But I think Moore's real point is simply that the Birth of the Nation does some of the same things as later superhero movies, with the KKK as the superheroes. They also happen to wear uniforms that are strikingly similar as it happens...
But it doesn't mean Siegel and Shuster were consciously -- or even unconsciously -- inspired by Birth of a Nation and the KKK. It might mean that there is a connection/commonalities in the reasons both forms were so popular.
Birth of a Nation was released in 1915. It was enormously popular, it was not some fringe thing, it was a mainstream popular success which generally did not have it's politics questioned in the mainstream, although there were protests by Black people. According to wikipedia, "It presented the Ku Klux Klan (KKK) as a heroic force." Heroic, eh?
Superman's first apperance was in 1938.
Every culture has heroes, and every culture tells tales of impossible heroics. It would be more accurate to call Achilles the first superhero, or Beowulf, or Gilgamesh. All of these characters are larger than life, and all of them are morally flawed.
American racists had racist, American heroes. That doesn't mean that Captain America is devoid of commentary about the nation's role in today's world. And if Moore had watched Civil War, for example, I think even he would be forced to agree.