On that basis I would say that the word of the United States is far more intrinsically valuable than any physical object.
But US dollars aren't guaranteed to do anything for you either in such a situation. The currency could hyper-inflate tomorrow for all anyone knows, and it has no intrinsic value other than the requirement to pay taxes with it. It seems to work fine as a currency and maintain stable value, but the point is it really isn't backed by anything.
Nothing, neither gold nor grain, is backed by anything but power. Fiat currency is just the government accepting that responsibility de jure.
Ergo, a dollar today is guaranteed, on the average, to buy just under a today's dollar worth of things next year.
Ergo, a modern dollar is about the closest thing to a guaranteed exchange value that the world has ever seen.
Being stable in value, and being guaranteed to have value are two very different things.
It is "backed" in the sense that people need it to pay taxes, but exactly how much value that gives it is completely uncertain and only determined by the supply and demand of people spending it/accepting it. If there were to be a major economic shift, the value could change to be literally anything relative to the value of other goods (without the supply and demand of those goods actually changing.) It could even completely collapse and be worth nothing under the right conditions.
If it was actually backed in the traditional sense it would be guaranteed to be worth some amount of some good or combination of goods, regardless what happens, regardless how many people are spending it or accepting it.