I have heard that..and tried it. I have no doubt it pulls the water out of the oil. I used a 50 lb. bag of rock salt in a 55 gal. drum. From my experience processing homebrew biodiesel, I can say that 50 gal of oil would 'yield' app. 20% glycerine...10 gal. I personally have a hard time thinking that a 2-3" layer of salt at the bottom of the barrel would pull and 'absorb' 10 gal of glycerine.
Perhaps there is ion exchange process going on...or some other chemistry that I don't understand....I honestly don't know.
Also...you then have another waste product...nasty salt...saturated with glycerine...if the process does pull the glycerine out. I have learned that cattle love glycerine, but only if you do homebrew biodiesel and use KOH...potassium hydroxide...instead of NaOH...LYE. Having had a few little tastes of glycerin, it is sweet. Cattle like salt licks...and they like glycerine...