The reason provided is so poor. Should have left it at not recommended with no reason.
600 pounds of tongue WH on ball on a RL will be worse than the average WDH owner (and dealer) adjustment. That would be adding about 800 pounds to the rear axle and 200 pounds off front.
In my experience and observations of full sized half ton towing, WDH becomes helpful around 400 pounds of TW. I cannot see how the RL would be any different. In fact, maybe more so as +800 pounds on an independent axle really changes the camber/contact patch which also changes handling on top of the weight transfer off the front axle.