I think he is arguing that you can't define an individual driver like that. Sure someone could drive stoned just as well as another person drives sober. There are a lot of fatal accidents every year involving sober drivers. Couldn't one construe that many can't drive sober and increase the level of difficulty and requirements to get a license.
When I was younger I was an idiot and I took my driving test completely stoned. The license picture was hilarious but I did indeed pass a government sanctioned driving test while completely stoned. My tolerance is much much higher now and although I wouldn't drive stoned because I have too much to risk I do think I could easily pass a government license test stoned. What does that say about our system?
It means you are smarter stoned than most people sober.