Is this really all that groundbreaking? I had a teammate in highschool remark that you could pinpoint in his grades the times when he started/stopped smoking weed.
Well not if you ask him as he would tell you his grades were highly correlated to his smoking habits.
Obviously its a non scientific anecdote, but my point is I thought it was well established common knowledge that marijuana affects cognition.
And his smoking habits might be directly correlated with reduced motivation, poorer time management, and any number of other things. Meaning that it might not be a direct cognitive effect, but a second or third order effect altering his performance.