I think I understand...
If Hellen had taken an actual picture of a gray card with the incident meter reading and the sun behind her the gray card would appear on zone v. Bright spots and shadows fall where they may.
If Hellen had taken the picture of the grey card with the spot meter reading, Wouldn't she have had to open up 3-5 stops to get the bright spot white and the grey card on zone V? Or do you use the long exposure and reduce development dramatically to get it all on a wide lattitude film. I don't see this working at all for color slides.
Thus there is really 2 or less real stops to create the same middle grey exposure. It might have been useful to take a reflected spot reading of a grey card from the camera position to determine the variances between the reflected sunlight on the grey card and the bright wet pavement.