Even if I worked for myself, I'd expect my efforts to reward me.
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 get the larger point the advice is supposed to make, but I'm sorry, I will never work for free.
I will quit working, if I ever become wealthy enough to afford it, and I would tinker on my own "for free", but I wouldn't call it work at that point.
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.
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".
But seriously, I'd set sail and never put a foot on land again. About the most work I'd do is rigging and some weight lifting. I would do that until the day I die, which with any luck would be sooner rather than later (being old and rich doesn't appeal to me almost as much as being old and poor). Does this mean I should look for jobs doing those things right now instead of in tech? No, that's silly. In fact, the best chance I have right now for ever being at the point in my life where I can do that is if I stay in tech and work my ass off for a decent paycheck.