Here's the thing; watts per square foot is a number that doesn't quantify what you're really after, which is light intensity in the plant active wavelengths, or PAR. PPfD is the measure of light intensity of PAR light over a given area.
Sooooooooo, why are watts deceiving? Because higher efficiency lets you get the same PPfD with less watts- which is exactly what these pioneering new ultra high efficiency setups are proving.
Put another way, his 238 watts clearly just produced results in a completely different league than a 250W HPS setup would manage. In the final analysis, efficiency- measured in PPfD/Watt- IS everything.