Thanks Joe. That's a code section I've been eyeballing pretty hard. I think they could make a somewhat solid case for 250.52 (A)(3) (concrete-encased electrode) if they were using a #4 or larger wire. They've specified a #6 though.
I've spoken with the architect about my concerns, and spent some time expaining the difference between grounding systems and bonding grids, and why I feel there's an issue with connecting the two in this manner. I think they're trying to go above and beyond the code's bonding requirement by doing this, but lack the fundamental understanding of what the bonding grid does. We'll see what plays out.