There are plenty of rewards that aren't monetary. The author isn't saying you should work in a job that provides no benefits. He's saying the benefit of money shouldn't be the primary one because other benefits (like fulfillment) are more important.
So the point of the mental exercise is to help you find a job that is worthwhile regardless of the money you're paid to do it. So you'd still be rewarded for your efforts just not in money (which again wouldn't mean anything to you anyway if you had a billion dollars in the bank)
I've got dozens of ideas that I've never had time to work on, because of long work hours and taking care of kids. I could work on those for years, easily. Some of them might become "work", if they caught on, but for the most part I'd be satisfying my curiosity and want to learn new things.
I think at this point, it's just a matter of conflicting definitions.
But yes, his advice only applies to independently wealthy people, unless you start attaching caveats: "If you wouldn't do what you do for FREE, then find another JOB, secure IT, and then QUIT your current one".
So the point of the mental exercise is to help you find a job that is worthwhile regardless of the money you're paid to do it. So you'd still be rewarded for your efforts just not in money (which again wouldn't mean anything to you anyway if you had a billion dollars in the bank)