Interesting, but far from conclusive science.
There are a couple of key factors not mentioned that greatly influence the composition of the skimmate being produced;
1) is the water being pulled directly from the tank or from a sump after going through a sock or other means of mechanical filtration?
2) what is the bubble/water contact time?
if pulling directly from the tank you will get much, much more organic matter in the skimmate than if pulling from the sump post filter sock.
contact time is probably the most crucial factor with overall bubble surface area being second.
Some compounds can take almost 2 minutes to be pulled out of solution. The longer the bubbles stay in the water/reaction chamber, the more compounds that can be removed.
Recirculating protein skimmers of the type used in the experiment do not achieve those contact times and attempt to make up for it by recirculating the water through the skimmer, but it's not the same as a prolonged contact time for each bubble.
I would love to see more thorough research utilizing different skimmer types and see what different results may be had between a recirculating skimmer and a tall counter-current type