As not everyone on the BB will have read Out of the Silent Planet, (that's clear from the question: "Lewis who?") it's important to state that it is a work of science fiction, as are Voyage to Venus (otherwise known as Perelandra) and That Hideous Strength, the other two books of the trilogy. So your quoted words were spoken by a fictional character (unless they were from the author's introduction).
You are correct ... my oversight.
I do think he poses a very interesting question ... what if there are thousands of planets with intelligent life, but earth is the only one that fell?
