I’m not trying to define the terms, just posit a very very simple theory of the form killing is wrong because human life is good. Such a theory would be inferior on its own premises than a very very simple utilitarianism, regardless of whether either or the premise itself is true. As such I oversimplified utilitarianism just as much, but it doesn’t matter for the scope of the example.
Edit: in fact, for the purposes of the example it is better if the “deontologist” is wrong about deontology, because it better illustrates how one theory can have greater explanatory power than another only on the grounds of the former’s justification without reference to external verifiability. “human life is good” is a poor first principle, but if it is true, the utilitarian’s principle applies it better than the “deontologist’s” did.
Here the deontologist is arguing for the principe ‘killing is wrong regardless of the consequences’ (deontic) but uses a poor justification for which consequentialism is a more reasonable conclusion. So the ‘deontologist’ is wrong even though his principle cannot be externally verified. I was just (unclearly I see) using this strawman to illustrate how theories could be better and worse at explaining what they attempt to explain without being the sorts of things which can be proven. I will attempt to be clearer in future.