The point I was trying to make is that if you're taking the view that your users need to have a significantly higher than average the level of knowledge in a domain area (i.e. equivalent to the knowledge of the domain area that the product designer has), you're going to end up with an un-usable product for the left end of the bell curve of your user population.

That left end of the bell curve is relative (as you point out) to your user population, but exists nonetheless - an "idiot" radiological engineer is not an idiot in relation to the general population, but is one in relation to the 99th percentile best radiological engineer (and, hopefully, in relation to the radiological engineer building your product).

What I was trying to say is that a well-designed UX requires that the 1% "idiot" of your target user population still knows what to do...