With an incident meter you get an exposure value that is supposed to be right both if the subject is dark or is bright. (This is probably not perfectly true at the "extremes" of pure white and pure black which made the OP raise the question, but that's irrelevant now).
The incident meter give you a value that will have the bright (more reflective) appear bright and the dark (less reflective) appear dark with that light condition. The bright details will appear bright because they actually reflect more.
With an incident light meter, if there is a middle-grey subject you should have it as middle-grey, but if the subject is darker (portrait of a black man for instance) or brighter (portrait of a pale white man, for instance) you will have the same reading and you don't need to "place" zones.
The incident meter will not give you a reading that renders a certain subject as middle grey but a reading such as a middle-grey subject will be rendered as middle grey, and a bright subject will be rendered as bright, and a dark one as dark.
Only when using a reflected light meter you have to keep in mind that what is measured (not necessarily middle grey) will be rendered as middle grey.
Hope this is not a too pedantic post
EDIT The point 2F/2F raises is that if the lighting ratio is low (similar lights) an average makes no damage, if the lighting ratio is high (quite different lights) an "average" will might lead to blown highlights. One controls the highlights with the exposure (pointing the lightmeter to the light source so as not to blow them) and the contrast with the management of secondary light. I find this to be perfectly reasonable and maybe this is also what Mark meant.