Wicked is built on the 1939 film rather than on Baum's novel, and the detail that proves it is the shoes. Baum wrote silver slippers; MGM made them ruby for Technicolor. Everything downstream, including this one, inherits the film's version. The story is a prequel to that film specifically: how the Wicked Witch of the West became wicked, why the Scarecrow, the Tin Man and the Lion are what they are, and what the Wizard actually is. It works as an argument only because the audience arrives already holding the 1939 film's account, which it then contradicts.
