that's very different from saying that biography can improve or disprove something! As a matter of facts, Nietzsche writes here about moral purpose of philosophy. "Autobiography" here is a trick; confirming that you must separate author life from texts. In German, Nietzsche uses the word "mémoires", which are an author own recollections of experiences. The "Auto" part in the translations, coming from the Greek autòs, meaning self, is the key part here. A description of Nietzsche's sickness made by someone else is hardly an autobiography.