It remembers me "The Mythical Man-Month" by Brooks. And in book you will find some sources of this myth.
Also I suggest to search for more recent discussions about this book, where you could see modern cases.
Maybe 60 hours a week if you aren't doing demanding work. Even with undemanding work 60 hours a week isn't all that sustainable.
> Does anyone know where he got this number from?
He may have just made it up.
Entrepreneurs forget that they worked this hard because it was their company. Most companies act like their employees should work this hard because of their culture. They forget that we wouldn't be there if they didn't pay us.
So I'll be working 60 hours a week at making myself superfluous and making a billionaire even richer than he already is. Yeah, but no thanks.
I've never read anything about a "sweet spot of productivity" involving working 60h/w. And it is kind of funny that his motto is not "work smarter" but "work harder" in the context of AI :-)