I'd challenge that very strongly. I think morality is very compatible with nihilism, both in the existential and moral nihilistic varieties.
In existential nihilism, the only real requirement is that the nihilist rejects any notion of intrinsic meaning to life and existence (at least if I'm not mistaken, I'm rusty on these things). Moral nihilism is a natural extension of that which just requires a rejection of intrinsic morality.
I would broadly say that it's really difficult to actually be a nihilist, just as I said elsewhere in this thread that it's hard to realistically disbelieve in free will, even if you intellectually process the arguments and agree with them. There's certainly a hard threshold to pass, but I disagree that it has anything to do with morality or principles. As far as I'm aware, constructing personal moral rules from first principles as a pragmatic matter is allowed so long as the nihilist 1) acknowledges that those principles are arbitrarily chosen and 2) not representative of, or descending from, any objective laws that can be gleaned from the world via deduction or induction.
Basically, I'm saying utilitarianism is a valid moral framework, and utilitarianism is compatible with nihilism. The nihilist will face significant challenges in actually living day to day life in congruence with nihilism (i.e. not really believing in anything), but the nihilist can still as a separate matter construct a moral framework that at least seems rational and internally consistent for the purposes of self-preservation.
Frankly this seems like sort of a rabbit hole. I can accept a stronger view of nihilism that disallows adherence to any moral framework, but how does that allow a nihilist to behave at all consistently or continue to function? If we take away the word "moral" and instead abstract away the terminology to, "a set of personally principles, consistently applied", what does a nihilist look like who simply doesn't have that?
If we go back to Rust, he does have moral principles, but his behavior and words are congruent with the core of existential nihilism: he rejects the notion of meaning in life and existence and admits that he attempted suicide in despair but simply couldn't out of cowardice. From that perspective it seems like nihilism plus a set of consistent principles could coexist as the continual dissonance between conscious hyper-intellectualism (existential despair) and unconscious biological drives (self-preservation).