The point of this test wasnt to bash clamping, it was really to show the accuracy of the AMM1.
Here is a breakdown of the test.
Amp Dyno is being used to provide a 4Ω resistive load.
On paper, the clamps, Dyno, and AMM1 should all agree on the same number since the load is resistive and the power factor is 100%.
Here is where the tricky part comes in. If you are "clamping" using just a general DMM, there is no way for you to see where clipping occurs. Now if you use a nice Oscope, you can now see the sine wave. BUT you are having to rely on your eyes to tell you where clipping occurs to find out what the numbers are at this point to find your usable power.
Once you take this into your vehicle and start using a subwoofer to create your load, now you run into phase angle which clamps cannot see.
Overall the AMM1 does everything for you and make it MUCH MUCH easier. It is an all in 1 tool.