How do you tell that a book with nihilistic characters isn't a nihilistic book? The difference is obvious in principle, but I'm not sure it is in practice.

I can't think of a single character in the book for whom nihilism is their defining trait, and certainly not the primary characters. The effort to preserve goodness in the world only really matters when it's hard, when it comes at cost. The book turns that up to 11, but that is why it is hopeful.

If you want to read McCarthy doing nihilism, maybe try the sunset limited.