Is Moral Nihilism a Morality?

[SenseMaker007]

But nihilists make the claim that there is no moral knowledge.

It's more accurate to say that they claim that the very concept of moral knowledge is completely meaningless because moral claims aren't addressing matters of fact (according to them).

Error theorists say the same thing: "there are no moral features in the world" and "all moral judgments are false"

It makes sense to say that error theorists are nihilists.

I think that philosophers have miscategorized Error Theory when they've said it's a form of moral nihilism because I think to be nihilistic about X is to say that X is without meaning ... and to say that X is false is to give it meaning.

I think what you say applies to general nihilists. But if someone claims that they are an ethical nihilist, then they can indeed have beliefs about x and y. The only thing they think is untrue is morality.

I don't think outright denying X is the same as being nihilistic about X.

Think of it like how strong gnostic/atheism is not the same as weak/agnostic atheism. Merely lacking a belief in God is more theologically nihilistic than positively believing that God does not exist. Do you see a flaw in that analogy?