The problem is that when we see that girl and "1983" we have no immediate reason to associate the two. Because we can't tell at all how old she is, because all Randall drew was a stick figure with some hair. Hell, it looks more like some guys I know than a girl. Apparently she's 26, but she might as well be 12 or 42 or 8 or 90 for all we can tell. That's why it's so hard to figure out what's going on until you use just some cold hard comedic deduction.
Even so, I still don't like it. It's the same sort of stupid playing with words that you get from sentences like "this is the first day of the rest of your life" or that dinosaur comic about a time machine that moves at one second per second. And when I read the alt-text ("She also starts every letter with 'Dear Future
But I do have one thing that is nice to say about today's comic: Usually, Randall Munroe only has nice things to say about girls and only portrays them in a totally positive, cool, interesting, funny, sarcastic, etc, light. This is so that real-life girls will like him more. But today, he called a girl annoying! Maybe not in so many words, but it's there. So that's nice to see.