You have to take into account that the population under study -- drivers who tested positive for marijuana -- are people who consumed some amount of marijuana AND judged themselves capable of driving. Some pot smokers will exclude themselves from this population because they estimate that they can no longer drive safely -- myself being one of them. So from this study you cannot conclude that, in general, smoking pot does not affect your driving skills.
Yes you can conclude that precisely because you don't lose the ability to know when you've had too much. Unlike alcohol, pot doesn't affect your ability to judge that.
