There is not one single cause for the onset of psychosis but many factors at play. Cannabis use can be a cause. Early interception of people at risk and treatment, does bring down the number of young people having a first psychosis.
This kind of article doesn’t offer any evidence toward the idea outside correlations - and this kind of correlation is very tenuous.
A good example to demonstrate the idea is the outsized use of tobacco products amongst those with ADHD diagnoses. Do we immediately conclude that ADHD rates are influenced by the availability of cigarettes? Or can we instead ask how nicotine may influence neural circuits that are abnormal in ADHD?
https://www.thelancet.com/article/S2215-0366(19)30048-3/full...
Next, I'm no statistician, but I have never heard of this kind of trial being proof of causality (it can only provide supportive data - but as above, we know that the results may prove incorrect in a stronger study design). There are a lot of confounding factors that make causality a difficult thing to prove in humans.
There is actually a way to prove causality in humans, and I suspect someone will do this for marijuana since it is a hot-button issue: https://archive.cdc.gov/#/details?url=https://www.cdc.gov/ge...