That is exactly what it means. When an addict loses their supply, they go for the next best thing. We have 20% of our population with  prescription for opioids. Prescription opioids have a misuse rate for that population of around 30%. How in the hell is that not  clear red flag? 4 in 5 heroin users started with prescriptions. How is that not  clear indicator of how prescriptions fuel demand?

It's impossible for prescription opioids to fuel demand if it's supply has been cut off from the user ... 

Heroin and synthetic opioids themselves are NOW the ones fueling demand for illicit drugs ... 

You can make the argument that prescription opioids could fuel future demand like I said before but it can not be the CURRENT problem if it does not persist being one since the substitutes are taking over ... 

Prescription opioids are the PAST but the PRESENT is heroin and synthetic opioids so we should concentrate on the latter ... 

Every year, new people start using drugs. Its not the same population. It exists as a cycle of people from the general population to the addict population and back again. Whether or not a problem gets worse or better depends on the flow of that cycle. There are two places to address that cycle. One is new addicts and the other is recovery. Our current issue is not a static set of users, it is the flow of people into the pool compared to the flow of people out of the pool. You are looking at this in a way that is far too static. 

Reducing the problem of prescription opioids reduces the new addict population. That means that less people are feeding the pool, which enables you to reduce the size of that pool. Every year, thousands of new people enter the addict population. This is a current problem. It does not suddenly become a problem once they start using heroin. It is a problem from the time they are first introduced to the drug to the time where they recover or die. We currently have 20% of our population with a prescription for opioids. That is fueling the pool. That is causing new people to start using heroin every day. Those users are getting addicted and growing the addict population. That is a very present problem. 

Prescription opioids are the past present and future. Acting like they are no longer a problem is the height of ignorance. They are a reason that people in the past turned to heroin, they are a reason people are currently turning to heroin and they are a reason why people in the future will turn to heroin. And they are also a reason why thousands of people are dying, so theres that as an added bonus. 

On the other hand, yes, we need to address the population that is currently addicted to heroin. That is why the first thing I brought up is fixing our recovery system. However, if you don't stem the tide of demand it is the equivalent of bailing out a sinking boat. If you don't plug the holes, the boat will just keep sinking.