John...I agree with your comment...
The reason for connection of the GEC within 5ft of its entry to the building is that there is less likely a chance that changes in the water pipe system to other than metallic will happen there than downstream from that point.
Once again I will say that if there is a real FIRE WALL separation between the 2 units they are considered separate buildings and a grounding electrode conductor connection at the building with the water pipe entry would NOT be required from the building without the water pipe entry from the street.
They are separate buildings with the fire wall period.
In my 9th edition of Soares Book on Grounding page 236 figure 13-10 depicts this same scenario.
Separate Buildings=Separate Grounding Electrode Systems.
