As a former psych academic and researcher, it ranges from annoying to outright infuriating when reporters rehash research, this one's somewhere in the middle. Mainly because most authors can't even reasonably give enough information to find the full article easily. This one hasn't been published on hard copy (yet), it's sitting in 
online publication so it can be edited for a future issue. The reporter decided to use the initial sample (2000) instead of the resulting sample (842) for whatever reason.
But this isn't news. The article conclusion more specifically said that "Cessation of cannabis use appears to be associated with an improvement  in capacity for recall of information that has just been learned. No  other measures of cognitive performance were related to cannabis after  controlling for confounds." Recall improvement after quitting is probably the most common finding in this kind of research.
Too many morons will look at this sort of research and foolishly think that scoring lower on something like a word recall test translates to a person being stupid (or the researchers themselves "THEY MUST BE BIASED" garbage etc., if it somehow puts the subjects in a better light). Because that's literally what the test is, they give you a list of words and ask you to recount them up to 30 minutes later, in another list with additional words. The only thing they found (assuming I understand their scale, not having access to the full text and I can't find the CVLT scale anywhere) is that heavy smokers who stopped got 1.5 more words ON AVERAGE than those who didn't quit. That's it. 
Stupidity isn't exclusive to marijuana smokers, everyone's an equal-opportunity idiot.