More of an extended comment than a full answer.
This is an interesting question at one level, but it loses some of its force if one disagrees with the idea that Nietzsche isn't really a philosopher. I can see how his output can be considered poetry or literature, but I disagree with the overall position.
"For example, Nietzsche's entire canon is a matter of what is true to Nietzsche - not what is true." This itself is a philosophical position - see Nietzsche's perspectivism.
Moreover, his contributions to the study of ethics and values in "Genealogy of Morals" and "Beyond Good and Evil" are definitely philosophical, especially the idea that morals undergo a genetic evolution and have historical basis, more so than any objective nature.
His idea of the Will to Power is a continuation of Schopenhauer's Will to Live, and has a definitely philosophical pedigree.
So the argument that Nietzsche isn't a philosopher is kind of difficult to maintain, if one sticks to a pseudo-Kuhnian sociological definition of philosophy along the lines of "Philosophy is the activity of professional philosophers".
More generally, I think the pendulum swings the other way: Even authors who are considered pure literature or poetry are still considered to have contributed to philosophy. They just did so using a different medium. Think of Voltaire, Dostoyevsky, Lucretius, etc.....
Update: Logical positivist would agree with the view that Nietzsche wasn't a philosopher, but on their account, neither were Hegel, Sartre, Heidegger, etc...given the strict definition and very narrow scope that they accorded to philosophy as "the handmaiden of science". See the IEP:
At the heart of logical positivism was a novel way of dismissing certain non-scientific views by declaring them not merely wrong or false, but meaningless. According to the verification theory of meaning, sometimes also called the empiricist theory of meaning, any non-tautological statement has meaning if and only if it can be empirically verified. [...], the logical positivists concluded that the bulk of traditional philosophy consisted in meaningless pseudo-problems generated by the misuse of language, and that the true role of philosophy was to establish and enforce the limits of meaningful language through linguistic analysis.