Let me approach this from a different angle. There's two parts here that you're conflating. You can assert what something is NOT before you have to assert exactly what something is.
Like
@WiolentOne pointed out, one is just observing that there is some kind of ethical standard. It could be anything from majority rule, to the golden standard, utilitarianism, a categorical imperative, UPB?... or any combination of the above that we haven't thought of yet.
Second, there's the part of being able to detail exactly what that standard actually is. I have no problem with conceding I don't know for sure what that standard consists of, but it's obviously not relativism, because taken to its ultimate conclusion it eliminates itself as a form of ethics.
So in other words, I can rule out a patient from having Karategener's Syndrome before I may be able to diagnose them with Hereditary Spherocytosis. Make sense now?