This is true... it does add a third party trust to the equation, which is never good. But they are supposedly stored in Brinks vault in Salt Lake City. Maybe one day, a decentralized crypto protocol will be able to weigh our precious metals and convert them into crypto immediately. Then we can truly all be our own central banks backed by real assets. Damn, that's a great idea if I do say so myself.
I would never trust a digital currency that is backed by a commodity stored centrally or decentrally. this is a recipe for disaster that introduces counter-party risk.
This has already been tested before with DGC, Liberty dollar , and egold. You can almost guarantee that the commodity will be stolen by the bankers, a government or third party or leveraged.