Posting for friend who has question about weight dist hitch

The WDH is needed because the hitch is not rated to carry the weight on its own.
The weight must be distributed to allow the hitch to work as designed. The truck sway system reacts to the truck and is not proactive with the trailer.
A trailer prone to sway is still swaying with a heavier truck the truck just masks the sway by manhandling it. Sway control and WD can be very useful even with a large truck.
An 8,000 gross weight trailer being pulled by the truck described in the OP post is still the tail wagging the dog.