The very first lecture of "Nietzsche's Zarathustra," not Nietzsche's "Thus Spoke Zarathustra," but the Princeton University Press book titled Nietzsche's Zarathustra, goes into discussing this to quite some detail, showing you how the ideas crept in, and the style of propaganda used then at the time to turn the Values of the ancient Greek on their head. The whole book is worth you time, even though it's completely off topic.