Interesting, but a possible source of error of all correlational studies (this discovered a correlation between memory and marijuna) is there are three possible cases. A causes B, B causes A, C causes A and B.
In this case, it could be people with bad memory smoked, or people with good memory choose not to smoke. Then there is the conclusion the article meet that smoking caused the memory. Finally, it could be an outside factor causing people to smoke and have bad memory or not smoke and have good memory. I would lean towards their conclusion, but there is still the room for that error.