PW are American and thus are embargoed from providing manufactured goods, parts or assistance to anyone in Iran without permission of the State Department. However, it would be difficult to stop something that was legitimately sold and used by someone in China being sold onto someone in Iran. There is no reason why someone in China would need a third country to move something to Iran. It didn't just come to South Africa for maintenance or improvements since the aircraft was placed on the SA register. I suspect that the transaction (sale or lease) was financed from South Africa. Since South Africa do not have sanctions against Iran and since American sanctions against Iran are not binding on South Africa the deal is kosher!
That said, how they are going to get PW parts is a mystery!