Is this really an 'alt text' what the author of this page is referring to? Isn't 'alt text' a piece of text that shows when the image is unavailable?

Technically it's a "title", however everyone refers to the XKCD text shown on hover as "alt text". (See http://m.xkcd.com/, even xkcd calls it that).

Hover over an image with an alt-text attribute, and a tooltip with the alt-text pops up. All of the xkcd comics have alt-text, and it's a good practice in general, for accessibility software.

Actually, as other people mentioned, the real attribute that XKCD is using is "title":

    <img src="http://imgs.xkcd.com/comics/aspect_ratio.png" title="I'm always disappointed when 'Anamorphic Widescreen' doesn't refer to a widescreen Animorphs movie." alt="Aspect Ratio">

Indeed!

Yeah, I thought about the same. The Internet is just the enormous knowledge base. And the Internet is not only WWW, it's also IRC, Usenet, FTP servers, TOR, all kinds of darknets. It's worth more than any printed encyclopedia or dictionary. Yet, there is still no a good way to find a particular information fast, especially for non-IT people. Therefore, every initiative to make simpler is worth attention.