RAD could be the worst condition in the world for a child to have. Better to have a terminal disease than an inability to love. I don’t doubt that a total lack of empathy and often morality contributes to a general decline in self care, and then thus makes addiction less of a boogeyman to avoid (because you just don’t care). I don’t think it specifically leads to addiction. As ruinous as it is to the individual and those who attempt to love or care for him.
Moreover if you have some of the oppositional, defiant features of the condition, you seek out things like addictions because it will just piss your parents off even more.
Sorry, but it does lead to addiction.
It is a persistent and consistently present pre-cursor in millions of lives to the setting up of the dysfunctional dopamine feedback loop, groove that is--essentially--addiction.
We are designed to seek comfort and avoid discomfort. That's extremely basic. The dopamine loop is a key part of that. Pain is a key part of that on the other end.
The existential pain of not being sufficiently loved IS A HUGE KEY to seeking other sources of comfort to ease the pain and provide at least some temporary bits of feeling better.
One can see such conditioning effects toward dysfunction and even substance abuse in very very young children.
It may be that the initial substance abuse is the bottle, or sugar, or some favorite food or treat. Transitioning to worse substances is a fairly straight route.