He may be thinking of the seconary cat in the mid pipe which is where most cats are in a single cat system. Disconnecting the battery only serves to reset the ECU, not erase it. The codes are still stored in the EEPROM. These can only be erased by a scan tool. Once the ECU reintializes itself, those DTC's that still pertain will turn back to the active list and force a MIL.
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