Yes, I know that one,
and it's kind of silly the way dictionary attacks make those far easier than they look (to people who don't realize
it is a comic).
I think most people don't realize what Randall was actually saying. People are lousy at generating entropy, but they still think they can do it -- so they do.
Given a string of x length, generated by either correct horse battery staple (CHBS) or pseudorandom characters (`strings /dev/urandom`), pseudorandom always wins.
You can pack a lot more entropy into smaller space. And your password manager is remembering it anyway.
But people being people, who tend to write things down on post-it-notes
the webcomic-not-security-analysis is making the point that you will be better served by CHBS than by exchanging "a" --> "@" ad nauseam.
Of course there are other rules too, chiefly the adage about the bear.
Also the one about (not) using MD5.