Cohen's d=-0.132 is considered a very small effect size. In other words: The effect is statistically reliable, significant, and disagreements between studies are very low, but not that big.

>The approximately 2-point decline in IQ in adolescent-onset frequent cannabis users is not to be clinically significant

In a population level this might be part of the explanation, but we would have to know the correlation between prevalence of use of cannabis and IQ decline in the population. In developed nations there is trend where IQ drops 0.2 points per year (2-points per decade) since mid 1990s. In developing countries IQ increases 3-points per decade as Flynn-Effect still works there.

Like any tool, it depends on how you use it. As a teenager, I used it to paint and write. It opened many doors. I found inner expression that was not possible otherwise.

The measures used in the study are not absolutes and only a fragment of what constitutes human intelligence.

Cannabis is better enjoyed as a late adult, maybe >25 years old, as almost any drugs. I wouldn't advise it to the youth, it can really ruin your concentration years later.

Rastas say that "cannabis" isn't for stupid people, which is an observational conclusion. There are other sometimes profound and imediate negative effects cannabis consumption

I feel it.