That is a very old general rule and doesn't really apply to today's standards. There are plenty of tow vehicles that weigh less than the vehicles they tow. I see goosnecks with two rigs on them all of the time that are being pulled behind F250 SuperDutys. The most important issues is brakes.
There is a HUGE difference between pulling a trailer and flat towing a truck. As a general rule when flat towing you strap the steering wheel to keep the towed vehicle from roaming behind you or turning opposite of you while backing. In doing this the truck being towed acts like a big plow when being towed thus pushing the tow vehicle in tight turns at speeds. as long as you take turns slowly it should not be a problem. I to have been in a f-250 pulling a 3500lb jeep and felt in being pushed around corners. That being said, brakes are definitely important and you need to have enough to stop the truck and towed vehicle!
