It isn't soy based insulation. This has been an issue long before any of the manufacturers switched insulation. This happened to my car in 1995. Car dealers in some areas see about 1 car a month with chewed wires. For reasons that are not entirely clear, rodents like to climb into the engine bays of parked cars.
The soy theory sounds plausible, but I haven't seen anyone present any actual data that shows rodents seek it out and chew it more frequently than traditional insulation. The attorney that led the class action lawsuit against Toyota admitted he didn't have any such information, but was sure something would come out during discovery. The suit was dismissed.