He's saying that there are so many white heroes and the whole culture of the "super men" in comics, that they might as well have been the "Birth of a Nation".
Nothing kooky about it.
It is kooky, and ironic, to imply that people dream of racial genocide solely because of the color of their skin.
Super heroes are described as that superior set of people, more advanced than mere humans (stronger, smarter, genetically advanced in many cases, and more relevant to boot). Heck, even the name superhero/supermen etc alludes to Nietchtze's and the Nazis' Ubermench.
They are also predominantly white, with good looks and physique.
That's the very definition of a super race...
If you are trying to suggest that the movie Birth of the Nation glorifying the KKK, or the KKK itself, have nothing to do with white supremacy and racial genocide... then we're just done here.
To be fair, Moore didn't really flesh out the argument either. But I definitely find it enticing. if you don't, that's fine.