I agree that martial arts might have some ameloriating effect on violent behavior but at the same time, those not exposed to T'ai Chi, Karate, etc. and who are exposed to messages from violent songs might be unduly influenced. There is no empirical evidence to support either case but to dismiss it out of hand is not looking at what evidence there is available.
Yes, I think that some folk songs are inciteful to violence and represent the violent tendencies in some of the cultures that they emanate from. I haven't noticed that all folk singers per se tend to be necessarilly free from violence. People react to music in different ways.
I would bet money that if you played an agressive piece of rock for a group of young male kindegartners, you would see a room full of hitting and pushing.
I think that people can be hypnotized into agressive behavior through song "messages". Maybe it doesn't always come out in predictable ways such as those who show no signs of overt agression but suddenly snap one day and shoot up people.
In short, to say that the study is nonsense, is nonsense. There might be some connection and that has to be determined by furthur study.