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.