It absolutely would. If 40% of people test positive for THC, then this would mean there is no effect. I find it unlikely 40% of people test positive for THC, but yes, it does matter.
(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...)
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.
And the study doesn't seem to differentiate between the different types of THC either, some of which are not psychoactive at all and which people use to relieve pain and anxiety. There's quite a lot of people using non-psychoactive THC which wouldn't impair driving.
It doesn't say anything about the distribution, only that the "average" (presumably, the arithmetic mean, a measure particularly sensitive to distortion by outliers) was at a particularly high level.
Can you explain what you mean by this?