The XKCD comic skips lightly over this by simply stating that the words were randomly chosen. But being truly random is actually hard for most people to do off the tops of their heads.

I doubt many people are taking away from that comic that they should use software to reliably randomly choose the words they memorize. Instead the advice seems to usually get shrunk down to "choose 4 random words," i.e. out of your own head. Most people don't carry 50,000 word dictionaries around in their heads. More like a few thousand. That changes the math considerably.