Actually they didn't say "mostly harmless," but I had to get that in there. What they said was
Occasional and low cumulative marijuana use was not associated with adverse effects on pulmonary function.
It should be noted that "occasional and low cumulative marijuana use" in this case means "a bit less than a joint per day, on average" so this is not looking only at your once-a-month tokers. Also, in pleasant contrast to every study I have ever seen that attributed horrible effects to cannabis (which are typically n=7, followed for a month or so), this study had a subject population in the thousands and tracked smoking behavior over 20 years.
As soon as I'd downloaded the PDF I looked at "Funding/Support" section, because I was curious to find out who'd paid for a cannabis health study that wasn't designed as a hatchet job. I was amused by the "Role of Sponsors," which noted
The National Institute on Drug Abuse funded this analysis but did not participate in CARDIA or review the manuscript