I haven't read the book Lewis is talking about so I have little to say about what exactly he means by devil worship, however as someone quite familiar with his work, though long since having left religion myself, I can say that he is not one use a phrase like that merely out of petty spite or to be pejorative, but rather because he thinks it's the best technical description, at least that would fit into two or three words, that reflects this is analysis of the book's themes. Lewis was a medieval and renaissance English scholar before he was a Christian, and his professional work is far the greater portion of his writing and bears at leaat as much on his thinking as did his popular apologetics.
Lewis's model devil, both by his own description and by its depiction when he uses such a character in his fiction, is a kind of well mannered bureaucrat who just happens to be made of such a nature that his sustenance must come from feasting on human souls, and the more miserable his victims are by the experience of being utterly deprived of their individuality by complete and permanent dissolution into the self of this demon, the more pleasure it derives from consuming them. It is essentially inhuman but has no connection with the conventional idea of a monstrous man-bat scampering around with pitchforks and goat themed headgear. Lewis even pointed out, in case anyone missed the allusion, that selfishness of this complete and all-encompassing sort is a territory adjacent to that of sexual desire and conquest, and shares some of its traits. Love becomes a demon when he becomes a god, etc.
The reason this is worth pointing out is that Lewis is reliably consistent with his basic criticism of the modern world as tending toward an excessive and unhealthy elevation of monumental individuality, to the point that some individuals who are in advantaged circumstances to assert their individuality at the expense of others', by outright dominating them, or depriving them of opportunity, happiness, or satisfaction, will certainly do so. and if unchecked will grow just like a cancer, for that's what they are: apex parasites grooming their hosts, suppressing their spirit(ual) energies and curiosity by anesthetizing them with banal distractions this decade, only to drive them into bloody trenches the next. All so that a few can live impossible, unsustainable existences at the pinnacle of human society, of a culture they literally cultivate to keep themselves in rareified and thoroughly selfish power. The portrait Lewis draws of Mr Savage in the Pilgrim's regress is probably the most thorough exploration of this idea, and Lewis in the voice of a character (drudge) who is given one of the more sympathetic, clear eyed and and omniscient perspectives (i.e. the authors own voice) in the book, speaks of him with admiration and even goes off to join his massing armies of Swastici and Marxomanni, "who are all alike vassals of Cruelty."