I will concede that it is not proven. There are other factors to consider, other variables that may be a factor. For example, are pot users more likely to be reclusive? And are reclusive people more likely to have a psychotic break? Are they more likely to play video games watching flashing images for longer periods of time than others? There are too many variables to control.
So indeed the pot itself is not necessarily the direct link. But it is a piece of evidence. These kinds of statistics are misused all the time. At best we can say that people who use high potency pot are 4 times more likely to have a psychotic break than not pot users. It doesn’t mean much, especially when we don’t know the rate of psychotic breaks in the general population.
That said, this is not the only thing we know about pot. We know THC alters the mind and passed the blood brain barrier. So the one piece of evidence proves very little. A lot more data needs to be read into the picture. It still may not prove the link, just help increase the confidence in the hypothesis by ruling out and ruling in other variables.
They DO know the rate of psychotic breaks in the general population. They kinda need that number to determine the increased risk of drug users.