One factor to be aware of: Michael Sullivan's big boom was happening at the time that Amazon was aggressively pushing its self-publishing arm. There were not as many kindle authors back then. The market was less crowded, and a good book with a good cover and good marketing text had an easier time topping a bestsellers list and riding the sales wave of the Amazon Marketing Algorithms.
Also, Sullivan apparently had a backlist of 4 books that he was releasing onto Amazon in quick succession, which generally results in better sales than the one-book-a-year model. Sullivan's next paragraph on the discussion forums (not quoted in the original link) is relevant:
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All that being said...most of that is in the past and it will be a long time before I have another project ready for sale. I tend to write "entire series" and only submit them to publication after they are finished. My current work in progress will probably not earn anything until 2015 and the only 2014 income I have on the horizon is self-publishing sales of Hollow World and whatever royalties the existing books might produce. Books tend to initially earn well then fall off drastically so I don't count on any "backlist income" if it comes - great.
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Which isn't to say that Self-publishing is not the more financially viable approach for many authors. In terms of income-per-book-sold, it simply can't be beat. I'd just be cautious about taking the particular numbers quoted here as being representative of the situation as a whole.