I would need to see more research on this subject to fully accept the conclusions of this study because there is one potential flaw: they may be pointing the arrow of causality in the wrong direction. They are claiming that using cannabis lowers motivation/dopamine, but it may just be that people with naturally lower motivation/dopamine are more attracted to cannabis use.
Like I said, I would need to see more research to confirm either conclusion, but my personal experience points me more towards believing that cannabis doesn't inherently demotivate people. Really, it just seems to intensify whatever you're doing. If you're a demotivated person who likes to sit around and be bored all the time, it makes being bored more fun. On the other hand, I know people who completely break the "stoner" stereotype and use cannabis to help with schoolwork, housework, various hobbies, etc.