I used to cycle home a lot when pissed as **** and never had any issues really, in terms of being all over the road or falling off. My point is for him to drive that erratically means he must have been utterly wrecked. Would it surprise me if he was on gear as well? Not at all but at the very least, when coked up, you wouldnt be making those mistakes after a skinfull so I believe he was "only" drunk.
As for leaving the scene, obviously from a moral standpoint its the wrong thing to do but as no-one was injured and the ban and fine would have been 10 times worse had he stayed, it was the right thing to do to get his ass out of there. A broken clock is right twice a day.