It's not the title, but rather a sort of "bonus joke" that pops up when you hover a cursor over the image (or long press on mobile I think). It's a staple of xkcd and other webcomics, but I'll grant that it does present some confusion for those who rely on alt text.
I for one appreciated that being in the post so I didn't need to click thru for the bonus joke as I usually would. Perhaps it's just better to call it something other than alt text.