The levels described are actually pretty low. The "legal limit" is so low for THC that anyone who's had THC in the previous days could test positive, even if they aren't "high" at the time of driving. It isn't quite the same as the BAC legal limits for alcohol. And it doesn't account for body weight, tolerance, and other factors that definitely contribute to how a driver reacts no matter how long it's been since they consumed THC.

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.