The above article states:
"The ground ring shall be installed not less than 750mm (30 in.) below the surface".

If the ground ring is supplemented with ground rods, does it still have to be buried 30"? Both qualify as grounding electrodes.

In my industry, i.e. refineries and chemical plants, we frequently use a #4/0 bare copper grounding conductor as the so-called 'main ground ring' with taps to equipment usually @ #2 AWG or #2/0 AWG.
To obtain the required resistance of (usually) less than 5 ohms, the #4/0 is typically supplemented with ground rods to form the overall equipotential grid. Is the #4/0 therefore, really a bonding jumper between rods? And if that's the case, is it no longer the 'ground ring' and can it be buried at a much lesser depth, of say 18"?