Jokes are inspired by taking off from the exact subject at hand, that’s what imagination is about, not staying in the box.
You have a point. It's possible that Munroe being a physist might've known about Balmer Jump... though that's still quite a stretch to assume it was the source of the comic.
> probably originated the joke
A completely baseless claim ... "probably" means > .5 probability, but there is no evidence to support that; it's sheer speculation.