Interesting study and it looks like a good one, but a few things to note. Sample size, while good for this kind of study, is not big enough. That's why metanalysis' or high quality reviews are more reliabe. Secondly, what was their definition of heavy smoking and which biasis were present and so forth? Lastly, it's a few years old, not that that necessarily means anything, but with conflicting data emerging it's worth considering.
Anyway, if we actually go into what the study, according to the article, concludes:
"
While cannabis may have an effect on the age of onset of schizophrenia it is unlikely to be the cause of illness"
So, according to their study it may effect the age of onset and it's unlikely that it can be the cause. Again, nothing conclusive. Furthermore:
“In general, we found a tendency for depression and bipolar disorder to be increased in the relatives of cannabis users in both the patient and control samples. This might suggest that cannabis users are more prone to affective disorders than their non-using samples or vice versa.”
So, they admit to finding a correlation between depression and bi-polar with cannabis users.
“Drug abuse is present more frequently in family members of all 3 samples compared to those of non-cannabis abusing controls. This is in line with past research confirming a genetic predisposition for drug use.”
And here they state that drug abuse was also more prevalent in relatives and family members of cannabis users.
Obviously it's hard to distinguish between causality and correlation, it's like the chicken and egg thing a little bit at the moment, but it's not exactly the endorsement you made it out to be.