Doesn't that support the hypothesis that high THC levels are dangerously impairing people's driving?

You need to consider other confounding factors

It is well shown that age (youth) is a major factor in accident rates.

Not necessarily, they could both be a comorbidity of some other factor (bad decision making causes both, for instance) but it certainly doesn’t refute it.

That was my first thought on reading it. Dope is for dopes!