The demon is not just lying at random - the demon is lying with the purpose of getting a certain reaction (in this case, getting the human to subscribe to the philosophy of materialism). The original quote is advice on how to use the human's cognitive biases against him, in order to better achieve that goal.
The point of the quote isn't materialism. That could be replaced with any other philosophy, quite easily. The point of the quote is that, for many people, subscribing to a philosophy isn't about whether that philosophy is true at all; it's more about whether that philosophy is popular, or cool, or daring.
The point isn't to mock the demon, or the materialist. The point is to highlight a common human cognitive mistake.
You correctly describe what the quote literally says, but there's a fine line between "I'm just writing a story which requires that these particular materialists be biased" and "I'm accusing materialists in general of being biased like this". The former is often a way for authors to hint at the latter without saying it.
I could easily write a story where the Devil tempts Jews into baking matzohs using the blood of Christian babies. I could then argue that I'm not really accusing any Jews except my fictional characters of anything, and that this is simply a story about how people can do bad things for bad reasons. But you would be completely justified in not believing me when I say that.
Another month has passed and here is a new rationality quotes thread. The usual rules are: