I used to think the same thing, but now I'm not so sure. NEC 250.64(C) permits sections of busbars to be connected together to form a GEC. Now I'm not sure exactly what the CMP had in mind with this language, and I don't think they were thinking subpanels, but with the exception as written I think you could argue that the EGC to the subpanel is also a GEC if a ground electrode is connected to the ground bar in the subpanel. That EGC would have to be sized per 250.66 though (#4 copper on a 200A service).
