More Words, Deeper Hole

Doesn't work that way. Copyright and trademark are separate things, and a particular work dropping into the public domain doesn't undermine the trademark holder's rights. You can put out a Blu-Ray of Max Fleischer Superman cartoons, you can even use images from the cartoons on the cover, but you can't commission your own original art of Superman, because that's still protected by DC's trademarks -- and trademark holders are actually required to defend their mark if they want to maintain their rights, which is why you get Disney suing preschools for painting Mickey and Donald on the walls.

I'd guess the comic is relying on the parody defense. But it's important to note that parody only applies if the derivative work is commenting on the original. If you stray too far and use the original work for general social commentary, you put yourself at legal risk.