Considering it was designed to be used specifically with aviation headsets and has the customer reviews from pilots of old ragwings to support it, I would NOT say it would work better with a walmart headset. Using lower ohm speakers will boost volume, but that's because they draw excessive power from the amp. A quick google for "impedance mismatching" will teach you more than you've ever wanted to know.
Could even be a typo in the manual from the real manufacturer that's used to building 8 ohm radios, or just a measurement standard like how 48 watts can be 12v@4a or 24v@2a.