The intention is that the random words are selected from a list of 2000 unique, common words.
Choosing a sentence is a different strategy, which is less secure.
$ wget -O ⅓Mwords http://norvig.com/ngrams/count_1w.txt
$ for i in `seq 10`; awk '/^[a-z]{3,}/ { print $1 }' ⅓Mwords | head -n 2000 | shuf -n 5 | tr '\n' ' ' && echo
videos possible disease maintenance chair
teen documents than without son
research interface library largest drive
___location ball beauty coming files
files middle fri meet air
guarantee samsung click super inn
legal previous rent resort use
reply thought better fresh phentermine
bad command once vehicle australian
fun random professor course sponsored
I'm not suggesting that 20 random characters is easier to remember, but for average Joe, it might as well be the same. Not only do they have to remember the words, the sequence, and how to spell them. Unfortunately we cannot expect this from users in general - the worst offenders write down a password like "rocket", so there is no hope that they'll try to remember a sequence of random words.
We shouldn't have remember passwords at all IMO. It's creating entropy by remembering things, but the human brain is inheritly bad at remembering exact things. Things like a yubikey is a better idea, plug it in, enter your pincode, and use a key pair to authentication. All the user have to do is keep track of the physical thing and the pincode.
Choosing a sentence is a different strategy, which is less secure.