Well, I think it is both.

Lewis described how humans can subscribe to a given philosophy independently of its correctness. That part is completely rational.

But he didn't use a random example to describe this bias. -- Just look at all other books he wrote, they all deal with the same topic. His choice of Devil is not the same as e.g. Tolkien's choice of elves. Tolkien wrote fiction, but Lewis wrote fiction as a propaganda tool. Tolkien didn't believe in elves, but Lewis did believe in Devil. -- Therefore it seems to me very likely that he wanted his readers to think about this specific example instead of using this kind of reasoning generally.

In other words, his work is an equivalent of a hypothetical LW article: "Top 10 cognitive biases that Republicans have, and how it influences their voting". (Assume that the author wrote dozen articles on Republicans, and none about anything else.) Cognitive biases: okay. Selective attention to one specific group: not okay.

Since the author thought materialism was thoroughly false

That's actually one of the reasons I'm more inclined to interpret it that way. If Lewis thought there were also materialists with superb reasoning abilities who believed in materialism by correctly exercising those reasoning abilities, I wouldn't think a passage about poorly reasoned materialists was meant to be a generalization.

this constitutes one of the mildest attacks on the character of materialists he could have written

Hypothesis: Lewis was well aware that there are materialists who are good people. Yet his religion forced him to think bad of materialists. He handled this cognitive dissonance by thinking bad of materialists in one of the weakest ways possible. In other words, he knew too much to be able to believe that all materialists are power-hungry maniacs, but he couldn't avoid at least politely thinking they were all materialists because of everyday human foibles.

It's like Lewis's beliefs about homosexuality. He was forced by his religion to believe that homosexuality is a sin and that all gay people should abstain from sex, but he tried to be as polite to them as he could within the confines of these beliefs and did not write about how gays are a menace to our children.