I would suggest that there are two perspectives through which you could view this passage. There is C. S. Lewis's larger understanding of the nature of evil, embedded across The Screwtape Letters and also across his larger body of work.
What you need to understand is this: for Lewis, evil is small, petty, and ultimately hollow. We see this in the characterization that emerges of Screwtape across the letters, who resembles nothing so much as a small-minded bureaucrat. Such characterization was intentional on Lewis's part. In modern pop culture, the Devil is so often presented as powerful and seductive, even glamorous. Lewis was deeply critical of this vision of evil, and his depiction of Screwtape is, in many respects, a response to this popular image.
So now, we turn back to the topic of your passage, which is found in letter 9. Screwtape's words here are centered around the subject of the physical pleasures, most notably those involving sex. Lewis is not puritanical in his understanding of pleasure. (Puritans would understand physical pleasure first and foremost through the lens of sin.) Quite on the contrary: for Lewis, evil cannot produce anything on its own but can only corrupt that which God has already created.
The physical pleasures are no different in this respect, and Screwtape advises his nephew accordingly: tempters must proceed cautiously when employing it to tempt their targets into sin. Pleasure can be a powerful avenue towards damnation, Screwtape certainly admits, but it remains a product of God's creation rather than the Devil's.