Earlier quoted context omitted.
Right, during a period in which pot use increased.
Sure, but it's entirely possible the schizophrenia-prone population did not alter their pot usage. Also, if there's a link to the amount consumed, the number of individuals smoking pot at all could have increased while the number of individuals smoking enough pot to trigger psychosis and schizophrenia stayed constant. Wider consumption of alcohol doesn't necessarily correlate with more alcoholism.
And it's assumed the distribution of smoking per capita didn't change by much. I think that's a good assumption; again, it's possible that what you suggested happened, but it's a bit sketchy.