If they do consider gold coins (you don't know, the desinformation fog is thick), the reason could be that they expect an attack on a paper money system, consisting of a competing gang of thugs printing false notes and spread them, draining resources from the country and sending the fiat into hyperinflation. It has been done.
Minting gold coins and spread them in the IS would not work, the extra coins would be just as good as the islamic coins. Making coins that looks like gold is not easy, and can be detected by anyone.
I think such an effort would work to dramatically hurt the utility of their coins. What they are issuing is a gold standard coin. It's value is based on it's commodity value and not determined like a fiat currency. That means that a fake coin does not contain the value the user expects.
While there are always ways to spot a counterfeit coin, it could be made difficult in a practical sense. The idea is to get shop owners, etc. to favor other currencies and not trust IS coins. That suppresses the value and reduces the income for the issuing authority. It would be a big operation however, and there are a lot of problems to getting them in circulation.
Well, this can happen with paper money, but not with gold coins, exactly because the gold coins will be valuable whatever amount is dumped. Excess gold money will just make them stronger, enabling them to buy more guns.