It's not as important that I don't agree where the joke is.  I acknowledge that the provided explanation of the impossibility of humming a fugue could be the intention, however I don't know that it is universally accept that this is the case.  Furthermore, as another posted above, explaining every joke in the song would fill up the page quickly.  It seems silly therefore to shoehorn in the explanations with footnotes.  This whole "present my knowledge of the reference this work is making" is the idea behind Trivia sections, and the current policy is to avoid that kind of content.