I don't think they are saying that anyone who fits their own system cant do it safely and possibly better that a professional company, but insurance companies possibly would only insure vans with systems fitted by a company.
I was talking to a women last week she works for an insurance company, if you tell them your car is parked off road they now ask for a photo of your car parked on the drive because people were telling them they have off street parking to get cheaper insurance when they dont. she also said things like dash cams, mobile phone holders that are hard wired into the cars need to be declared as classed as a modification to the car. so I suppose if your system is hard wired to your van battery then you would need to declare it whether its bolted down or tied down with string. she said that if not declared then your insurance wont cover you.