The only thing he could mean that would make sense is what Diapositivo said. Stevie Duncan must have been referring to where the loss of texture occurs on either the high or the low side of middle gray (I'm not sure which side he was talking about), but not to the entire dynamic range.
Even so, 3-2/3 stops seems like quite a lot of headroom above middle gray, or below it, with Velvia. I would say that the film (ISO 50 version) "feels" to me like it has perhaps 5-6 stops dynamic range at most. That is an informal judgment, and I may draw the line for the edge of texture in a different spot than others do. But IME, Velvia 50 really is just a stop or so better than Fuji FP100C, which is very narrow-ranged (about 4-5 stops, informally judged by me). Velvia 50 is pretty much the only film I make use of instant prints for, using FP100C. The Fuji instant material gives you a pretty fair idea of how the tones will fall on Velvia 50, though the FP100C is a bit more contrasty.