EGC's would not carry any current under normal conditions. A GEC connected to the water pipe and then directly to the service neutral would allow neutral current to flow on the water piping system under normal conditions. That seems to be the reason for their objection to connecting the GEC to the pipe.
If there is no GEC, those EGC's ultimately tie into the grounded service conductor and electrically are connecting same places together as a GEC would, so any concern of currents passing through the piping and causing corrosion or electrolysis are invalid, same currents can potentially exist.
But also as already mentioned DC currents are what should be a concern for electrolysis and corrosion.