No, actually a currency is not backed by a nations assets.

It is backed by the obligation to pay your taxes in that currency. I.e. every citizen with tax obligation (i.e. basically everybody) needs to collect Dollars to pay his tax bill. So everybody is willing to accept payments in that currency or in a currency which can be reasonably easy converted to Dollars.

As a topper sales tax creates a tax obligation on virtually all transactions within the society, so the need to own Dollars is directly bound to economic activity.