I completely agree, that it more than one study needs to be done for any level of conclusion to be reached. I also agree that it has a lot more to do with the person than just whether they smoked pot. However, that has little to do with the study. The study simply claims a decrease in IQ. There are certainly successful people who don't all have top level IQ's.
I interpret it as more about the level of success you set your self up to achieve. A successful person will be successful. However, a successful person who becomes addicted to marijuana and starts smoking everyday may very well still achieve their goals and reach a satisfying level of success, but the point is unless done with discipline and thought, the habit of smoking can easily become a hinderance that decreases your chances of succeeding at whatever it is you choose.
Person A smokes on weekend, or vacations, maybe a few times a week, when it is convenient and won't disturb other parts of his/her healthy, successful life. It can be positive to relieve stress, relax and enjoy. It may damage IQ, but is managed well so that is all it can damage. Ultimately it may be a positive.
Person B smokes everyday without discipline--before class, after class, when they wake up, whenever the urge comes. Smoking easily interferes with most aspects of their life and becomes a hinderance to success. Basically, smoking comes first and results in a spiral of poor judgement and vision.
This is generally from my experience. I basically went from person B to A. Im not saying that frequency that you smoke is the issue, just whether or not it is properly managed or whether a person allows it to interfere with their success.