You fail to also consider the fact that you will be able to increase clock higher at the same voltage.  While the power draw is linear the system load is static so you improve overall SYSTEM efficiency if not card efficiency.

Alternatively you can improve stability at lower voltage for the same clock.  A 5% further reduction in voltage would result in a 9.75% reduction in power.

You also have more control over the waste heat (put radiator outside for example) and that means bringing down ambient temp at the PSU intake.  cooler PSU = more efficient PSU which further improves system efficiency.

If climates where AC is needed, dumping waste heat outside can reduce true cost of mining by 30% (or more for lower efficiency AC units).

Finally it is small but fans pull up to 6W each at full load.  That is 18W on a 3 card rig.  While radiator may need fans (although fanless radiators due exist) they tend to be larger more efficient fans (CFM per watt) resulting in a drop there.

Now if he said I can improve efficiency 70% with watercooling your "outrage" would have been justified.  His statement is accurate.  The savings may be small but the most efficient miner enjoys larger profits when price/diffculty is low and survivability when price/difficulty is high.