Kwired, not sure where you are getting that from. I am not seeing anything in there talking about "normal operating current" let alone grounded conductors.
Maybe a little reading between the lines of what "other than those established by grounding and bonding connections" means.
To me it basically means that a secondary conductor should not carry current of the primary system, whether grounded or not and the only electrical connection between primary and secondary should be via grounding/bonding conductors and that they should be arranged that they won't carry normal operating current.
Now throw in separately derived vs non separately derived generator system - the SD arrangement we switch the grounded conductor in the transfer switch because if we don't we can end up with stray currents on parts that were not intended to carry current, wiht NSD arrangement we still have two sources but we arrange things so that there is still only one neutral to ground bond point and that keeps non desired current from following non current carrying components - in normal operation. If there is a fault condition we still want current to flow, and hopefully enough it will operate an overcurrent device rather quickly.