>must not consist solely of lowercase letters

Which is exactly the sort of terrible restriction xkcd is criticizing.

A space is not a lowercase letter, so the xkcd password would pass my test.

Then the space would be "the obeisance to the stupid website piece". Note that the entropy of "correct horse battery staple" is only one bit more than "correcthorsebatterystaple".

Yeah, a lot of my passwords look like "securesecretpassphraseA1!"

There's the secure piece, and there's the obeisance to the stupid website piece.