These aren't anecdotes, it is a sample of 3801 individuals comprising those who used cannabis for different periods and different quantites and those who did and did not receive a non-affective psychosis diagnosis and/or report at least of CIDI hallucination item, to which were applied various statistical analyses that accounted for prior conditions and family history.
Those who did not use cannabis at all were excluded. As the paper says:
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It could be argued that siblings discordant for cannabis use (ie, one sibling who had never used cannabis and a sibling who had used cannabis for several years) may differ in a range of factors that could impact both the exposure variables (ie, propensity to use illicit drugs) and subsequent mental health. Thus, we undertook an additional planned sensitivity analysis where we restricted the sibling pairs to those who both used cannabis. This analysis allowed an even greater focus on the critical nonshared exposure (ie, duration since first cannabis use) and the psychosis-related outcomes.
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As a result, it is the degree of exposure to cannabis that is being analysed, not simply the use of it. The findings are that the longer it was used, the greater the chance of psychosis or psychosis related outcomes.