They're saying that if 23 states had legalized heroin then the government would have intervened to stop them. So the fact that the government hasn't intervened

They're saying that if 23 states had legalized heroin then the government would have intervened to stop them. So the fact that the government hasn't intervened shows that it isn't as dangerous as other schedule I drugs, and therefore shouldn't be schedule I. (Even though as you say cocaine and methamphetamine are both legal by prescription, and heroin is basically legal as well in the form of morphine, but whatever.)