Followup: I talked to the guy in the field, and he told me that the concern was coming from the definition section of the Code about separately derived systems. The Code defines it as "A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system."
Huh? The language about having "no direct electrical connection, including a solidly connected grounded circuit conductor" makes no sense to me. Should the grounding conductors be connected, or not? In our case, should the ECG of the inverter be connected to the grounded center of the Wye?
It seems to me that our inverter is not a separately derived system, anyway, because its output IS directly connected to the service. Am I missing something?