The problem is that the optional deice system on caravans is not designed or approved for flight into known iceing, it is a 'get out of jail' system however it does give pilots flying illegally into icing conditions the confidence to try and fly through or climb out of trouble. This trap has caused the demise of many cessna caravans in the commercial (cargo) world - but may not have anything to do with this incident.
Caravans are certified for flight in known icing conditions. There are several AD's dealing with the vans in the ice however. Basically, get the hell out if you find yourself in the ice.