There is definitely some impairment long term. I think we can assume that even without this huge study. I don't think knowing that matters unless we have some scale that tells us how dramatic that impairment is.
If I compare someone who smoked all the time, but also had a rich and engaging social life with a lifelong abstainer who live a mostly isolated existence, who is going to be more cognitively capable?
There are so many inputs that feed into who we are and what we are capable of that, in all likelihood, cannabis use is likely only incidental to much larger factors like isolation, media preferences, home life, etc.
As an anecdote, the worse thing I think I've ever done for my brain, the thing that I immediately felt dumber after doing, was reading "Shadow Wolves" (Steven Segal's book). I had trouble communicating thoughts effectively for weeks after finishing it. It was like it had sucked vocabulary out of my brain. Maybe that's how cannabis leads to long term impairment. It makes you read "Shadow Wolves".
Edit: Since people are voting this, I want to add an extra warning that I was not joking about "Shadow Wolves". If you read it, you will feel dumber, and it will take time to heal the damage done. Seriously.