Earlier quoted context omitted.
That isn't what the article was saying. I do find it a bit unfair to compare daily smokers to functional alcoholics, however - there is a definite difference in the way the drugs affect folks. This was the main point of the article. A slight buzz like that of a daily smoker - or someone using for medical reasons - isn't affecting safety, but going more than that and it should be obvious. Be safe and all that. Besides…
The key point in the article is actually the tests used. THC stays in the body long (days/weeks) after you're no longer impaired. If a breathalyzer test on Tuesday picked up the beer you had last Saturday, there'd be no correlation between that test and accidents, either. (and no one would approve of using such a test for law enforcement).
For alcohol, a comparable test might look at long-term changes in liver biochemistry associated with alcohol consumption. In that case, there probably wouldn't be any correlation with accident rates.