Basically, yes. It's fentanyl residue in some level of the chain after multiple steps of purchase-cut-resell.
At some layer of the long winding black market route, people will be cutting drugs with any sort of lacing to increase profits, they might be dealing with multiple different kinds of drugs in high volume and use fentanyl to cut their heroin/opiates to get more potency out of less product. When using some tool that wasn't cleaned properly and still has residues of fentanyl (which has a ridiculous high ratio of potency vs dosage) it ends up getting mixed into other drugs.
At some layer of the long winding black market route, people will be cutting drugs with any sort of lacing to increase profits, they might be dealing with multiple different kinds of drugs in high volume and use fentanyl to cut their heroin/opiates to get more potency out of less product. When using some tool that wasn't cleaned properly and still has residues of fentanyl (which has a ridiculous high ratio of potency vs dosage) it ends up getting mixed into other drugs.