Dennis,
the GRII is not an insulation. It is a test to evaluate the use in a specific condition. The product is still XHHW-2, it is just evaluated and listed ( i.e the GRII) as such and placed on the conductors insulation. In fact we are complying with UL requirements and they permit the marking of GRII to be added to the cable.
We do not design wire and cable to the NEC, we design it to UL Standards and ASTM Standards and so on.
This is also a good example of when an inspector (and I was one) requests specific documentation from the manufacturer to explain anything they might not understand. However, the electrical inspector has to also learn other documents as well outside of the NEC and the UL Standards are a good place for them to start.