I think the main difference between the PoCo and a structure is: the PoCo grounds the neutral in multiple locations but does not run a parallel ground wire. So, applied to the scenario described above there would be one ground rod at the transformer, one at the panel (ridiculous if the 2 are only 6 feet apart, but as far as PoCo distribution is concerned the distances are larger of course) and one at the panel but only the neutral wire connecting to both of them. A neutral-ground connection after the point where a separate ground wire is derived can lead to all sorts of trouble, especially downstream of a GFI or RCD.
