I think you're taking the wrong point away, which is that the causal implication here runs from "wide availability of a substance -> addiction to that substance

I think you're taking the wrong point away, which is that the causal implication here runs from "wide availability of a substance -> addiction to that substance". It doesn't say anything about whether a traumatized group will or won't take to substances; it just denies that all addiction is the result of trauma. The other open question here - I assume you're referring to remote Indigenous communities in Australia - is whether people use substances strictly out of trauma or also out of boredom. The degree to which Indigenous lives are rendered both traumatic but also incredibly boring is shocking. If you've gotten a low quality education, don't have a job and your traditional way of life is no longer viable (it's quite likely you haven't been taught it), how are you going to pass the time?