That document is quite confusing as it shows the earth going through the RCDs on the diagram, your first impressions were right imo FF, unless you know what you are doing banging stakes in is pointless, for example and I wont bang on too much, a crap ext reel of 25m plugged into the mains indoors will probably give you an earth reading of lets say 3.50 ohms which would be quite poor I agree but an earth stake using the ext reel live as one conductor (R1) as a fixed given and this temporary stake as the return (R2) would be very unlikely to be less than 200 ohms, probably more like 500 so the flow to earth is still going to take the ext reel path as its much less resistant, if that makes sense?
Generated power is different because YOU are providing the earth at the stake put in near the gene, your stakes around the site then use the actual grounds conductivity to flow back to the main stake, earth itself is a confusing term because the electricity only flows to earth on a fixed electrical supply because the energy suppliers have put earth plates in the ground at each sub station.
Easiest way just use an RCD on the front of each reel, if 30mA of electricity does not return via the neutral it turns off regardless of where it went that it shouldn't have