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.There is some really interesting research being conducted by Dr. Earleywine out of SUNY-Albany that looks at the link between substance use (particularly marijuana use) and the development of schizophrenia and/or Cluster A "eccentric" personality disorders (paranoid, schizoid, schizotypal). He tends to focus more on substance abuse, but I know that this is a major component of his lab. Def check it out if you are interested, I've heard AMAZING things about him as a mentor/advisor.
(I can split this out if people want to talk about this in more depth)