Steve, separate electrodes simply must be connected to the grounding network; supplimentary electrodes, however, require a full size grounding electrode conductor.
In other words, if your 'ground wire' were #6AWG, and run without splicing all the way back to the service disconnect, you MIGHT have a hook to hang the argument on.
BUT- such a 'hook' would be a creature of grammar, not engineering. Remember, the 'ground rod' is there for lightning, and not for clearing faults. A separate structure is a separate target. Wouldn't you want to keep the lightning out at the shed?
Now, we can split hairs all day about what is, or is not, a separate structure. Without trying, I can think of several layouts that blur the line. I think the code simply uses the 'one branch circuit' rule as a threshold, a way of saying 'more electric than that, and it is, without a doubt, a separate structure.'
