You don’t get whiplash from being hit by a tailgater. The difference in speed when hit by a tailgater isn’t very much, a few MPH. Its when there is a big difference in speed that you get whiplash, i.e. 30 MPH or more. You don’t develop that much of a speed difference when you slam on the brakes with a car only 20 feet behind you.
I have had three major hit from behind, two of them resulted in fractured vertebrae in my neck. These were not from tailgaters, they were from people well behind me who simply didn’t stop. One was intentional, she rammed the guy stopped behind me at abut 30 mph and caused a six car pile up. Road rage, but they didn’t have a name for it back then. Another was a guy who suffered from an epilepsy attack behind the wheel. The third was on drugs.
But I am not defending tailgating. The idiots who slam on their brakes for no reason are just as bad. Two wrongs do not make a right.