For one thing, it's highly convenient that the standard US work-week happens to be the exact "correct" amount of hours to work to maximize productivity.
For another, anecdotally at least, most people know plenty of people who work longer hours and do it successfully. And most of the really successful people out there will mention that they worked insanely hard at some point. Are they all lying? All wrong?
The article itself mentions people doing extra hours of coding on personal projects, and we all know some people who have started companies that way. What, do these projects simply not exist? How does that make any sense?
You can think it's exploitation to ask people to work hard, but this is to work on a team with a world-class researcher who most people would love to work with. Is it too much work for some people? Sure. Are there others for whom it is worth the trade-off? Probably. Why does everyoªne on HN just assume they know better than those people themselves, and set off to make them feel bad and exploited? Working really hard is a totally legit and rational choice under some circumstances.
(Btw, worth pointing out that this article is at least partly a pitch for a book about working less hours. Not saying this invalidates its points, but it is certainly taking advantage of the publicity Andrew Ng's post has gotten to sell a product).