Intuitively, this makes sense. Anyone buying heroin/meth/whatever would've likely also been exposed to the option of buying marijuana just as easily among those alternatives when it was still completely illegal. If marijuana was a viable alternative for their drug of choice, I imagine they would've just stuck with that regardless of legalization, since it's cheaper and safer.

Hence I wouldn't really expect legalization of any drug X to improve problems with any drug Y in most cases. But if legalization itself helps with drug problems in general, I might expect that only legalizing drug Y would help drug Y's problems.