I also thought the study was convincing in showing a significant correlation*
It's been folk belief for a long time that pot makes you slow, unmotivated, and dumb.
Therefore, I can't rule out that the study failed to control for giving a shit about one's mental abilities, a factor that's nearly certain to explain the outcome by a sum of all possible intermediaries, of which pot is just one.
[*] even though the number of people in the 8+ IQ point losing population seemed to be only about N=30. 1-2 IQ points difference is probably not significant, but I'm guessing N was larger, so maybe. Also, the same data set was mined for genetic correlates with heavy pot use, so they could have easily controlled for that (could turn out that e.g. the same factors cause persistent pot use and adulthood cognitive degeneration, though I judge that unlikely). Since there is so much demand from the DARE set for a finding like this, you also have to expect quite a lot of publication bias and population-cherry-picking, as well.
A week ago the meetup group in Berkeley discussed a new article in PNAS titled "Persistent cannabis users show neuropsychological decline from childhood to midlife". Several people who didn't attend said they were interested in how that conversation went.
Before discussing the specifics of the article, we went around the room and stated how many IQ points we'd be willing to spend for some level of cannabis use. The median answer given was 4 points for moderate usage. Then someone pointed out that since we responded out loud, there may have been an anchoring effect here.
My understanding of the group's understanding of the scientific result is that smoking so much marijuana that you're diagnosable as cannabis-dependent (whatever that means) before the age of 18 will give you an IQ hit of 9-11 points, maybe more, over 20 years, compared to nonusers.
People who were diagnosable as dependent on cannabis but not before 18 got an IQ hit of 4 or 5 points, on average. We don't know if this is because cannabis is bad for adults, or if it's bad for people just over 18.
People who have used cannabis but were not diagnosed as dependent got an IQ hit of 1 or 2 points on average. The article gives us little information on what the risks of various moderate levels of cannabis use are.
We didn't discuss any methodological errors in the study, but the general attitude of the group was that the scientific result is worth taking seriously.
After the discussion, people who use cannabis opportunistically or not at all — especially the younger attendees — said that after learning about the study they now have another reason not to use cannabis. One person who uses cannabis less than once per week said they wouldn't change their usage habits.