I guess their equal entropy stems from them being all common words and not just random strings of various length.
Ie a common word is an atomic entity here, like a letter, or a number and their total amount is limited.
He may have counted all common words and determined that picking from one of them (in all lower case) is the entropy equivalent of an 11 bit number.
That means he's made the assumption that there are only about 2^11 = 2048 common (english) words.
I dunno if that is correct. Maybe he only counted 'common' words with a certain length... You go check.