In this sense it's a "truth" because it works and therefore is "true enough." Functionalism.

This is not my understanding of nihilism though. Although Nihilists say there are no objective truths and therefore no objective meaning to reality, they still privilege objectivity as the ideal type of truth or meaning. Thus they see culturally relative or individually subjective truths/norms/conventions as less valid or real than the idealized but nonexistent (according to them) objective truths that they apparently yearn for.

I say this because of all the "woe is me" "God is dead" whining associated with nihilism - the realization that meaning is created, not discovered, should be a cause for celebration, not despair. It means what we do matters.