That's Robin, disguised as an autograph hunter (he also got Joe DiMaggio's signature in the previous panel), claiming that Superman's not real. Later in this issue (and a different story), his cover will be selling Batman and World's Finest comics, so now I'm confused. Was Superman created by Siegel and Shuster, or are they just putting out Superman comics based on a real hero in their midst?
Now what "famous restaurant" is this so we can all line up for a sig?
Still, I'm open to other suggestions. (And I know the question wasn't posed in full seriousness.)