Rick I'm not trying to advocate for one system or the other here, just for understanding what they do.
Yes, the incident meter assumes that "the light it's in" integrates or averages out to middle gray, but like you also said "systems don't make decisions, photographers do". Regardless of the metering method we use, we have to ask ourselves "what am I measuring" and factor that in to find what we might consider a normal camera setting. From there we have to decide if we want it to render normal or high or low.
BTZS users factor in what they are measuring when a shaded reading is taken as the basis for setting the camera exposure. That concept is also used when they find the difference between their shadow and highlight measurements to help them decide on their EI and processing.
Dunn uses that concept when Duplexing, which he did with a flat face incident meter.
With regard to different skin tones and spot metering those faces, as long as the placement offset to a "normal" camera setting is "a known" it doesn't matter what the specific tone is, this is a true zone system concept and equal in practice to incident metering. With an incident meter we don't need "a known" target (skin tone or whatever else) in the scene to find the same camera setting.