The study doesn't say that cannabis was being smoked, just used.  It is very clear about smoking cigarettes, so that's an important distinction.  The study wouldn't use those distinct terms if cannabis was smoked by all participants.  Cannabis is taken in many different ways by many people, and a large amount of therapeutic cannabis isn't smoked.

The link is probably not because of delivery method, it's far more likely that it is because they are both plant derived.  All plants store chemical components in similar ways because of their very structure.