That wouldn't actually mean no effect, you need 40% of people driving to test positive for it to be no effect. It's unlikely the population driving is equivalent to the population at large - for one there's a set of responsible people who won't drive while high. For another weed use isn't randomly distributed through the population but correlated with certain subsets, which probably have a non-average rate of driving just by coincidence.

(Not that it really matters since I don't buy for a second that anywhere near 40% of people/people-driving are high at any given time. I also don't put much faith in numbers in the abstract of a a yet-to-be-published study...)

There is a case for the two populations to be quite similar.

THC in the blood doesn’t mean actively high for habitual users, which would be most users if THC consumption is high. It means recent use, but not clear impairment.