While salt certainly repels moisture, sugar can actually soak it up, drawing it out of the meat. They even state that the sugar draws out the moisture to form a molasses that coats the ribs. Unless the moisture was coming from another source, there is no other way, without being heated first, that brown sugar would take that form. I've never eaten ribs cooked this way, so it could be a possibility that the molasses forms a coat which actually seals in the remaining moisture, making them more moist. I plan on trying this on some BBs this weekend to see how it works out. I'll probably do six hours at 225 though.
On the salt thing though... I watched an episode of No Reservations with Anthony Bourdain (I think, could have been Andrew Zimmern) where they completely covered a fish in sea salt to keep the moisture in while they cooked it. Im not sure where the line is drawn as to when moisture is withdrawn or sealed in when using salt. Ill let someone more scientifically inclined weigh in on that one.