The GPS method is obviously measuring the total system error. GPS ground speed measurement is highly accurate; much more accurate than position measurement.
It's easy enough, on a nice day like yesterday, especially doing it over the ocean and on autopilot, to fly at a constant and accurate IAS, and fly constant headings.
The effect of flying a heading that is very flightly off (say a few degrees) has a negligible effect when using the three-headings method.
The thing which puzzles me, not understanding pitot tube physics much, is how one can just stick a plastic hose to the front of a pitot tube (and another one to the static vents) and declare the ASI has such and such error.
The aircraft was a TB20.