I read that quote as saying that the Devil (or a demon) deceives people by making them believe those things, not that the Devil believes these things himself. That's how demons behave, they lie to people. This one lies to people about why one should be a materialist and the people fall for it. The point is not to mock the demon, who in the quote is acting as a liar rather than a materialist, but to mock materialists themselves by implying that they are materialists for spurious reasons.

Of course, Lewis has plausible deniability. One can always claim he's not attributing anything to materialists in general--you're supposed to infer that; it's not actually stated.

Edit: Also, remember when Lewis wrote that. 1942 wasn't like today, when it's possible to say you don't believe in the supernatural and (if you live in the right area) not suffer too many consequences except not ever being able to run for political office. Any materialist at the time who claimed he was courageous could easily be just responding to persecution, not claiming that that was his reason for being a materialist. Mocking materialists for that would be like mocking gay pride parades today on the grounds that pride is a sin and a form of arrogance--pride in a vacuum is, but pride in response to someone telling you you're shameful isn't.