ROSPA are doing a bit of scare-mongering, but what they say is sort of right. If someone were to have an accident of some sort due to their negligence in some respect - say they dropped their phone and was trying to retrieve it from under the seat while they were driving. When being questioned about it under caution, rather than admit their stupidity, they try to mitigate their negligence/ recklessness by saying that the reason they had the accident was that they were put off by their loud music. In that case, the police would have clear grounds for prosecution.
It reminds me when I was young a friend had a motorbike accident because he was messing about with something and ran straight into the back of a parked car. Rather than admit his stupidity, he came up with the excuse that he must have blacked out. He didn't get prosecuted. Instead, they took his licence off him on medical grounds. IIRC, it was a minimum of two years before he could reapply, and when he finally got the all-clear from his doctor, he had to explain to the insurance companies about it, so his insurance went right up as well.