Remember, the salesman has only one person's interest...HIS! All he cares about is making a sale so he can earn a commission. Listen to the people here. They have decades of experience and only want what will be an enjoyable RV experience for you.
A weight distributing hitch transfers weight from the tongue to the front wheels of the tow vehicle as well as the trailer wheels. A properly set up hitch will actually put downward pressure on the front and rear springs of the tow vehicle.You can actually measure the front and rear fender heights before and after hook up and should see a difference.
So, your salesman may be technically right that some of the hitch weight may be transferred back to the trailer, some would be transferred to the front wheels, so he is exaggerating the results.
I don't remember a discussion in the previous thread about battery weight on the tongue of the trailer, making the actual tongue weight 50+ pounds higher than the published weight.