Great answer, Greg!
Another quibble I have with the artwork is that it appears to show a GEC that is larger than the service 'hot.'
Again, you're never required to have a BEC larger than the 'hot.'
I'm not sure you're required to have the GEC 'land' in every panel (or disconnect). As I see it, 1 meter = 1 GEC, land it in whichever panel is convenient.
No? I'm thinking of the meter bank at the apartment building across the street .... 12 meters, one GEC landing in one place.
Using Holt's explanation (ground rod for lightning / static only), I see the issue also as one grounding electrode for
each building. I don't see the pictured 'tap' as accomplishing anything at all.
As for lite bulb's second question ....
TWO sets of 3/0? A parallel feed? Well, that's a different way to do it ....
First, I'm uncomfortable feeding the panels without having a disconnect first.
I'd run 500's to a trough above the panels (as in the drawing), and tap off then with 3/0's for each panel (using NSI's).
Could you run your GEC taps up into that trough, splice them together and run the GEC from the trough to your electrodes? I don't see why not- assuming the trough remains accessible (not sealed by the PoCo).