Why does it have to be insignificant? How often are ISIS and related groups found to be trading in weapons? Smugglers would easily be able to dispose of Bitcoin if they wanted to use a more traditional route. Bitcoin could be held by them to fund future expeditions if they believe that Bitcoin can keep on rising.
It is insignificant, because that is what the ground reports indicate. The ISIS seems to be having much better payment options to purchase their weapons. Most of their weapons come from the Turkish and Arab cartels, and these people mostly accept United States Dollar, or bullion (gold). Sometimes weapons are purchased in exchange for crude oil.