I think they pointed out that based on their experience, it's probably not even possible to separate correlation from causation cleanly and I think I agree. Even though I never smoked I spent a lot of time around people who did and I could have told you even at age 14 before they took their first puff that they were, on average, much more likely to exhibit some of the things the study listed as "findings" like "more likely not to have a job", etc.

This is more or less the nugget.

What makes a teenager year old seek out pot/ alcohol/ whatever.

If a study about long term drug use doesn't address why people started smoking/ doing drugs, then a critical piece of the puzzle is missing.

I think GP’s underlying point is that the study is observational and can only determine correlation not causation.

I think he's trying to say the stronger statement that not only does correlation != causation, but he also believes there are known confounding factors that weren't accounted for.

That's ok, it's not like I'm paid to do this.