> of which the PI even calls for reproducibility
You mean like every study I've ever read? I can't recall the last time the conclusion didn't say something like, "In our study X appears to be associated with Y, but more research is needed to understand the relationship."
You're right, though, that this article leaves many questions unasked. We know that people who are schizophrenic are far more likely to smoke cigarettes, but there is evidence to suggest they're more likely to smoke before their first episode as well. So is tobacco causal? Probably not. Instead, there are probably precursor symptoms to diagnosable schizophrenia that drive tobacco use: anxiety being the main one. Anxiety and marijuana has already been studied with conflicting results, probably because it's hard to determine out whether people with anxiety are drawn to marijuana (or heavier usage) or whether heavy usage causes anxiety.
There are reasons to think marijuana use can cause schizophrenic episodes - sometimes a first episode - but that use may not increase the risk of developing the disorder when viewed in a 20-year window. In other words, it happens sooner. So according to the article the number of schizophrenia cases linked to cannabis use disorder increased by 4x. I'd like to know whether schizophrenia diagnoses overall changed significantly.
I am not saying there are no risks. But I am agreeing that the article does a bad job of analyzing the science.