Lewis's Essay "On Three Ways of Writing For Children" ... describes the Boy's Book or Girl's Book with its immensely successful schoolboy or schoolgirl who is able to perform incredible tasks with little or no preparation beforehand. Harry Potter and his magical abilities fit into this implausible category, and for the most part, so do his Quidditch skills. As has been noted, beginning with Price Caspian but also to some extent in The Lion, The Witch, and The Wardrobe, Lewis avoids creating this type of protagonist.
What's wrong with the Boy's Book or the Girl's Book? (Think of the Hardy Boys or the Nancy Drew series, where the protagonists are barely old enough to drive a car but somehow are able to fly planes and sail submarines, and the adolescent heroes spend their time catching international spies* rather than scooping ice cream, babysitting, or mowing lawns.) In discussing this type of book, Lewis points out "We run to it from the disappointments and humiliations of the real world: it sends us back to the real world undivinely discontented. For it is all flattery to the ego." The pleasure consists in picturing oneself the object of admiration."
When we get back to our own lifes in the real world, they seem less satisfying than ever. While we may dream of scorring the winning goal in the final moment of the championship game, this kind of dream may cause us to despise our real lives and the world we actually live in.
Two quibbles. The Harry Potter-type books also rely on feeding the idea you are secretly very special, even though ignorant others don't notice it. Someday. Someday... I would also like to point out that as naturally gifted as some of these protagonists are, they are nonetheless portrayed as having also worked to become as skilled as they are. And Chip Hilton did scoop ice cream. Yet even so, very few are remotely capable of being such a multisport star, however hard they work.
*The international spies in these books have never been taught that special pugilistic trick of disabling your opponent by hitting them in the solar plexus. This turns out to be an important omission in their training every time, because the Hardy Boys, who presumably learned it from self-defense books you can order from the last pages of a comic book, defeat them with it repeatedly.