Is he arguing their is a pre-existing condition that would lead to SCZ/related and the people are using/abusing as a coping skill, or that the substance use can some how impact the development of SCZ/related? SCZ isn't my area, but I believe there is some good support for substance use/abuse with people who have SCZ, as for many it is the best way they have learned to cope, though I haven't read anything on substance abuse impacting SCZ. There have been a few studies that looked at LSD, MDMA, and cocaine use as possibly factors in experiencing a psychotic-like episode, though I don't believe there was anything definative.
(I can split this out if people want to talk about this in more depth)
There's been some research on the possibility of early and frequent cannabis use causing schizophrenia, but the field isn't conclusive yet. Here's a review:
Cannabis Use, First-Episode Psychosis, and Schizotypy: A Summary and
Synthesis of Recent Literature
Michael T. Compton, Sandra M. Goulding and Elaine F. Walker
Abstract: Schizophrenia currently is conceptualized as a neurodevelopmental disorder, the expression of which is affected
by both genetic and environmental factors. Schizotypy is a dimensional personality construct that is the substrate of
schizotypal personality disorder (SPD), and its positive, negative, and cognitive features are subclinical manifestations
that mirror the symptoms of schizophrenia. Research indicates that both schizophrenia and schizotypy are linked with
cannabis use. In this overview, recent literature on the following topics is summarized: (1) cannabis use as a potential risk
factor, or component cause, of schizophrenia, (2) prevalence and clinical correlates of cannabis use among individuals in
the early course of schizophrenia, (3) clinical and psychosocial outcomes of schizophrenia that are influenced by cannabis
use, and (4) associations between cannabis use and schizotypy. Implications and future research directions are discussed.
Growing evidence points to the complex and informative interconnections between cannabis use, schizotypy, and firstepisode
psychosis.