I think you misunderstood the comment. It could be better written as "By the time they leave school they can already be..."
Now if a kid watching videos that are long form treatments on specific subjects and are following allong with a pen and paper doing practice problems, sure, thats a case where youtube might give you some serious skills, but that is a tiny fraction of kids and those same kids would have been doing the same shit with textbooks at their local library. Youtube is waaaaaaaay closer to being purely a distraction than to being something we could consider "educational" in any serious sense.
The thing is, when people are motivated to do something, they'll learn what they need. Otherwise, not even school can force feed students into learning those things.