The study:
“The Effects of Alcohol and Cannabis Use on the Cortical Thickness of Cognitive Control and Salience Brain Networks in Emerging Adulthood: A Co-twin Control Study”
Does not mention schizophrenia in any way.
Here’s a study on that topic:
Cannabis use and risk of schizophrenia: a Mendelian randomization study
...”This adds to the substantial evidence base that has previously identified cannabis use to associate with increased risk of schizophrenia, by suggesting that the relationship is causal. Such robust evidence may inform public health messages about cannabis use, especially regarding its potential mental health consequences.”
https://pubmed.ncbi.nlm.nih.gov/28115737/
MR studies are very flawed purely statistical manipulation and do not show direct differences like one can absolutely see in twins if trying to test for true causality.
https://www.nature.com/articles/d41586-019-03754-3
Here is a more recent larger twin study of causal effect on schizophrenia of approx 3000 individuals.
https://psycnet.apa.org/doiLanding?doi=10.1037%2Fabn0000701
“We found no evidence of an effect of cannabis on Psychoticism or any of its facets in co-twin control models that compared the greater-cannabis-using twin to the lesser-using co-twin. We also observed no evidence of a differential effect of cannabis on Psychoticism by polygenic risk of schizophrenia. Although cannabis use and disorder are consistently associated with increased risk of psychosis, the present results suggest this association is likely attributable to familial confounds rather than a causal effect of cannabis exposure.”