The concrete itself is the conductor. Rebar is lapped and ties for 48x the diameter to be considered "continuous" and it is felt that the physical contact and the concrete itself is enough to establish a ground reference for the grounding electrode system. Fault current itself is send back to the utility via the bonded neutral, not the GES. The GES is really only there to establish a ground reference and mitigate transients, not fault current.
250.4(A)(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that
are grounded shall be connected to earth in a manner that
will limit the voltage imposed by lightning, line surges, or
unintentional contact with higher-voltage lines and that will
stabilize the voltage to earth during normal operation.
