In the chloride cell the nickel, copper, and gold would ionize and then just plate out (or be reduced together) on the cathode, so your not separating the metals.
In the concentrated sulfuric cell, the depletion of electrons of base metals such as copper at the anode is more limited, making removing the gold plating easier without attacking the base metals as much, the base metal ions, the base metal atoms that were oxidized (dissolved) into the ionic form will stay in solution when the electrolyte is diluted as sulfate salts of the metals dissolved into solution. The gold plating the gold atoms are only temporarily oxidized (only a temporary loss of electrons from the atoms of gold) dissolved into an ionic form upon losing electrons at the anode, and again the gold ion is soon reduced back into metallic gold as soon as the gold ions leave the anode area on their journey towards the cathode, basically giving you a way to separate the metals...