I couldn't disagree more strongly with the claim that it's "unfortunate" for people to assume, in the absence of strong evidence to the contrary, that drugs associated with impaired cognitive function probably don't mix with activity as potentially lethal as driving.

Especially since the balance of the evidence you've linked to appears to support that assumption.

It's not as if "there is at least some level of marijuana use incompatible with driving" is exactly an extraordinary claim; the burden of proof for establishing parameters for a "safe" level of high whilst in control of a vehicle really ought to sit with proponents of smoking and driving.

Put another way, jumping to the conclusion that driving under the influence of marijuana isn't safe isn't likely to kill people if it's incorrect.