And where we had thought to find an abomination we shall find a God.
Where we had thought to slay another we shall slay ourselves.
Where we had thought to travel outward we shall come to the center of our own existence.
And where we had thought to be alone we shall be with all the world.
I hope my answer doesn't state the obvious... but I think you are correct in seeing this quote as the "hero's journey" in a general sense. He was bringing up the idea that the battles and journeys are ultimately about the self: understanding, accepting, conquering... and eventually seeing the divinity in knowing/accepting who we are.
The reason this is so significant in Superman (and Batman) is because we like to make Superman (and all superheroes) as perfect gods fighting the evil "out there", but the idea is that they, too must see that evil and divinity is not outside of them to be conquered. One of the things we like about these comics is that they are constantly struggling with their own humanness. Superman needed to realize that for him to become a true Superman his greatest fight is with himself not with these other enemies... So to wear the costume he needed to acknowledge this.