This is a little like arguing that someone else's meaning of life is incorrect. There are different philosophies, and those that argue being able to recognize you have enough is important have been accused of being selfish. I remember a conversation where someone was appalled that that was how Pope John Paul essentially wrote off Buddhism in one of his books.
The author is the real deal in the sense that he did choose to decide he had enough, and set up a foundation to support music education and began advocating for entrepreneurship.[1] Maybe the reality is that Derek Sivers does not believe that he could as effectively manage a larger foundation as he can manage a smaller foundation like the one he set up. You might not make the same decisions as he did. I wouldn't, and I disagree with the extent to which he seems to have taken "mo' money, mo' problems" to heart. However, you could also say that his quality of life is maximized by him being the one who makes those calls, which may be a suitable localized goal, maximizing global benefit more than if he were striving to his limit in supporting more ambitious goals.
[1] https://sivers.org/trust