Posting for friend who has question about weight dist hitch

OMG. Just looked at the owners manual of my new 3500 diesel single rear wheel. Owners manual says 1500 pound tongue weight? Oh that just drives me insane now, because I don't understand how a 1200 pound weight distribution hitch should be needed at all? What am I missing. I was following this thread very clearly, under the assumption that my tongue weight was rated at no more than 1000 pounds. Now that I know it's that much higher, it's very unclear to me now :(

Even the owners manual says WDH optional