No, backing implies that someone has a legal obligation to supply something in exchange for the thing backed. Gold originally backed USD because paper dollars

No, backing implies that someone has a legal obligation to supply something in exchange for the thing backed. Gold originally backed USD because paper dollars were literally a contract to pay the bearer a certain amount of gold. Gold and USD now have the same sort of relationship that USD and bitcoin do: neither USD nor bitcoin is backed by anything. Both are still worth something, at the moment, though, which is all you need for an exchange to be possible.