Well... I can see how it can be easy to reach conclusions like this. The same result (most likely even worse) would be reached even if the effect of alcohol was studied instead of cannabis. This is not surprising, but neither is it interesting in my opinion. Of course there is a correlation between time spent doing mentally engaging activities and success, and surely smoking weed is not a mentally engaging activity. But this does not imply anything about moderate consumption in leisure time, neither does it mean that we should never engage in activities in which we do not exercise our brain.