The weight is well above the truck center of gravity, so having it shift suddenly can make maneuvers very interesting.
He is dead correct. VERY INTERESTING.
Diesel fuel has a weight of 6.951 pounds/gallon on the low end. You are talking at least 347.55 pounds (50 gal tank) transferring to the outer radius of the turn all at the same time. That, multiplied by the distance above CoG, along with tongue weight of the trailer makes one hell of a mass shift. The less the mass shifts, the less effect there is on control. Having the weight that far above the Center of Gravity (CoG) can actually cause the truck to roll over in emergency situations.
This all depends on tire inflation, other weights and their placement, suspension components and their condition and a myriad other factors. Best and safest is to have baffles in the tank to prevent slosh.