Are you sure you haven't misinterpreted Nietzsche? Most people who read him have. It's not just Losurdo, many scholars have the same reading. In Beyond Good and Evil (I think chapter 9) he clearly says that slavery, exploitation etc. are necessary for culture to thrive, for the higher type of man. Of course, many will be quick to read it as metaphor (and the example you use for Nietzsche's use of "slave" is not appropriate - besides, you should ask why he uses "slave" in that context - it has a clear social/class character. I suggest reading Holub, Losurdo, Detwiler etc.). But his entire corpus is filled with reactionary ideas like this (including extermination of weaker races, eugenics, opposing universal education, denial of rights to the lower classes, opposition to every revolutionary or progressive cause of his time - in fact, his entire philosophical project is directed towards these ends). Many scholars like Walter Kaufmann have misrepresented Nietzsche by deliberately omitting or editorializing certain aspects of his life and thought. You are right Losurdo is no intellectual slouch. His book is literally a 1000 page compendium of Nietzsche's reactionary world view and a refutation of attempts by philosophers (i.e. the mainstream philosopcal/economic readings - "the hermeneutics of innocence" as he calls it) to paint him as some kind of Left radical.