subtracting within reasonable bounds the effects of luck and privilege
That sounds like an overwhelming exception to me.
Yes, it is an overwhelming exception. In the real world these differences always exist, and you'll have to use your intuition to correct for them.
I'm trying to make the least convenient possible world where two randomly selected people are pulled from a crowd and are given the same, luckless task and one does better. Existing differences in brain-biology, priors, and previously gained knowledge still apply, while differences in resources and non-brain-related biology should be factored out. In these unnatural conditions, when it comes to that specific task, the one who did better is by definition a superior instrumental rationalist.
Related to: What Do We Mean By "Rationality?"
Rationality has many facets, both relatively simple and quite complex. As a result, it can often be hard to determine what aspects of rationality you should or shouldn't stress.
An extremely basic and abstract model of how rationality works might look a little something like this: