I think the assumption is that he'd have more sales if it was sold by a publisher. Sure, he would have. But he shopped the idea and it was rejected. Even if his sales quadrupled, it probably wouldn't have been a book that traditional publishers would have been looking to publish.
Selling 1800+ copies of a book no publisher would touch is an achievement, and not an easy one to reproduce.
If you can come up with a really good idea 4 times a year and follow through to completion within a reasonable deadline you're only looking at a $36k income. That's not very good as a full time job. However - if you can do that in the evenings it's a heck of a side income, and the more you can consistently perform, the more you'll sell.
There's also the question of investment. Amazon's options are zero or minimal investment. Spend a little bit of money and have 100 or 1000 copies printed at a time and handle the delivery yourself and you can double your profits - plus you have the capability of handing over copies to local booksellers to see if they'll sell in store.
For every person looking to go down this path, there are a lot of paths you can go down. The easiest path and most potentially successful is a publisher. They have marketing plans, distribution contracts, and above all else - talent on hand who are pretty good at determining the sell-ability of a given book. They aren't the only answer, though. 2000 copies sold is nothing to a publisher, but for an individual it can be huge. Personally, I think that before writing anything you have to determine your target market and how to communicate with them.