Dear Fred,
Can't speak for Sandy, but yes, incident light reading does indeed rely on an artificial highlight.
For landscapes, in glaring sun with clear air, 1 or at most 2 extra stops over a full-sun reading should give adequate shadow detail: guessing how much extra exposure to give is a matter of experience.
Now consider 'double lighting': sun streaming through the window of a church, where you also want detail in the dark wood beams in the hammer-beam roof 20 feet above your head.
An incident reading somewhere in the church, a reasonable distance from the window, would probably (though far from certainly) give adequate detail in the beam. This is essentially BTZS and is a good fudge, but it's still a fudge.
A much better idea is to take a shadow spot reading, to determine how much exposure you need, and a highlight spot reading to determing the actual SBR (not the SBR inferred from incident readings) and therefore the gamma you need. Then a gamma/time curve (ever harder to find nowadays) will give you your dev time...
Cheers,
R.