I think in the agreement it also said that PG&E can choose to dump excess power into the battery. There is also supposed to be a setting to limit how much PG&E can draw down on the battery but no mention of disallowing PG&E to dump into the PW2.
The ITC only cares how your charge the batteries. I would say that allowing the utility to discharge your batteries into the grid to support the network would be a type of VPP. However, I would personally only agree to it if there were specific compensation for every kWh fed out in that manner.