I probably shouldn't even comment here because I don't know the subject matter at all, but for what it's worth, it strikes me as a pretty desperate joke on a pretty desperately obscure topic.
Reader ZZ points out that Jefferson had little to do with the Second amendment; he wrote the Declaration of Independence but was traipsing about in France when the Constitution was written. Nice try, alt-text, but your history is flawed!
Did any of you get the joke? If so, is it any good? On a related note, can anyone think of a more obscure xkcd? This and this strike me as close, but they were both very early ones, and probably drawn when he was in class learning about those topics.