""The dual action (or random-orbit) sander just makes the scratches less obvious.""

Yeah that is basically what the guy was saying. I would have tossed the opinion but then i thought about it too much...the depth of 220 grit scratches
by hand and by machine may be the same but by DA sander the scratches are blended and therefore the result is similar as a higher grit paper by hand?
At the molecular level the scratches would be closer together and more blended with a DA sander..same as with a higher grit by hand?

I think this one is close to the "if you put a plane on a belt that speeds up as the plane speeds up will the plane ever take off. ..except i know the answer to that one."