I've heard similar before but there is no way on Earth that my 2.5 tonne Ford Territory will stop quicker than my 300 kg ST1100A. The car might have better braking capability but it has a weight disadvantage.
If a car can stop quicker than a bike (emergency) then an 18 wheeler should be able to stop quicker than a car because it has 18 wheels that have brakes and more rubber on the road.
Trouble with theses tests is they don't actually use a bike and a car to compare but I will agree that a rider and experience make a difference as well.