Bitcoins are no more a commodity than US Federal Reserve Notes are, and nor are they backed by any fixed commodity standard.
But the dollar is a commodity.
No, it is not. The US dollar is a fiat currency, and nothing more.
And, by the way, what backs gold?
Historicly the yellow metal was prized for it's beauty in jewelry. In our modern world, there are many uses for it in industry for which it is ideal even though it is too expensive. For example, lead is poisonous to humanity, but it's mass number makes it both a wonderful material for making ship ballasts and radiation shielding. Gold has been used for both these things in the past, but only in very specific situations.