I feel like that is a government-sponsored myth.
It’s a JOKE, dude. But it does come from a grain of truth. I can attest from personal experience that alcohol impairs your judgement, which, with regards to driving, will influence speed and recklessness as well as reaction time, while marijuana will leave your coordination and reaction time intact, but affects how your brain processes information. So, you could stop at three intersections in a row with traffic lights, and reach the fourth that has a stop sign, and just sit there because your brain is waiting for something to change to indicate that it is time to go.