The sentence you quote is inevitably going to be true for any type of Raft quorum that can reach consensus with a minority of nodes. You don't even need to get into the specifics of the math.

Suppose you have a quorum Q. Then its complement Q' must not also be able to form a quorum; if it did, a network partition between Q and Q' would create a split-brain. So if Q is a minority subset, then Q' is a majority that cannot reach consensus on its own.