On the other hand, I don't get out much. For all I know, Doctorow might be famous for being an avid balloon pilot in addition to a writer, and that's what everybody are alluding to. On the other-other hand, I cannot seem to google up any non-humorous Doctorow/balloon juxtapositions.
So perhaps it is all an in-joke referencing the xkcd strips, or perhaps xkcd was itself referencing some ur-joke known to only a few initiates (or, alternatively, to everybody but me). Impossible to know, really – when an Internet meme reaches a certain critical mass, half of those who pass it on have no idea what it refers to, anyway. Close to nobody ever played Zero Wing, but that does not prevent the all your X are belong to us snowclone from being productive.