IPC Code makes no requirement as to distance from the floor. Only that it must discharge in a "safe manner" The universal standard here has always been within 6" and most plumbers install it in that manner. The NMC specifically states 6" for relief valves installed on boilers and pressure vessels. I think your inspector got it wrong.
504.6.1 Discharge. The relief valve shall discharge full size to a
safe place of disposal such as the floor, outside the building, or
an indirect waste receptor. The discharge pipe shall not have any
trapped sections and shall have a visible air gap or air gap fitting
located in the same room as the water heater. The discharge shall
be installed in a manner that does not cause personal injury to
occupants in the immediate area or structural damage to the
building.