This article is nothing more than an advertisement. The company that makes freelanceswitch also owns the sites listed: FlashDen, AudioJungle, ThemeForest, iStockPhoto
The author clearly mentions this in his post, so I don't see why that is an issue.
I think the issue is that AFAIK it is not that easy to make $2000/y with selling stock photography. Actually, it is a hard, persistent work. So he was propably just lucky, so they picked him up to advertise them.