Wait a minute. The claim was against marijuana use, not any other substance abuse. Plus the screening does not eliminate undiagnosed abusers using marijuana or any hard narcotics to self medicate for even years before being diagnosed with schizophrenia. That whole claim is ludicrous. The researchers would only be looking at an individual after the fact, not the individual's prior history.
The actual study sliced and diced the data for multiple types of drugs, primary mj users is just one category.
What they are saying is that for all users of any type of drug who had a substance abuse disorder diagnosed up to one year before being diagnosed with schizophrenia were not counted as drug abusers for the purpose of the study under the assumption that they were self-medicating prior to clinical manifestation of schizophrenia.