First, if he used and then drove while still under the influence, then it's driving under the influence which is the same as if someone had had a few drinks and then was driving.
Second, the keyword is "when". Metabolized THC will stay in your system at detectable levels for a couple of weeks (depending on many personal factors). So theoretically, he could have smoked a joint two weeks prior and still tested positive for THC at the time of the accident.
Well, that would be deaths attributed to marijuana rather than the nonsense of 0 deaths. As far as that specific case, there is circumstantial evidence that he had in the past smoked dope will driving with kids. Weed may be more harmless compared to heroin or cocaine, but it is far from harmless.