I don't think that's true and that should be easy to prove: the books you will read in GCSE English are below the level that any kid who likes reading will have read independently - yet no-one worries that it is turning kids off reading.
That seems surprisingly generous towards GCSE English. I know one straight-A student that I went to school with that swore he would never read another book in his life after leaving his Higher English exam (Scottish equivalent of A-level or whatever they are called these days). I'd imagine that if you weren't already a reader then it wouldn't have fired you up to become one either.