You’re correct to note that the U.S. was never on a pure commodity standard. The various monetary regimes under the U.S. Constitution, the bimetallic standard, the classical gold standard, the gold exchange standard and so on were all influenced by government intervention, including legal privileges supporting fractional-reserve banking. This is the reason that booms and busts have occurred throughout U.S. history.
It is worth pointing out, however, that the inflationary potential of the different monetary regimes has been steadily increasing, which has been the intention of the federal government in bringing about the monetary regime transition.