That line is available to any other central bank in the world without going through primary dealers at all - it's just that China is using it to avoid giving in

That line is available to any other central bank in the world without going through primary dealers at all - it's just that China is using it to avoid giving info of its UST holdings or buying/selling trends to the open market. This is especially important now as China has widened the Yuan trading band. You wouldn't want excessive FX volatility to manifest itself as a result of your foreign-reserve management decisions being (mis)-interpreted by investment banks.