I'm not certain what your question is, exactly. If you're asking if those "keyword density" numbers are correct, you'll find if you read around the forum that around here we generally consider "keyword density" to be an artificial (and useless) metric. I suspect few to none of the mods and admins would agree with the quote you provided, for instance.
It's just a part of what I often call paint-by-numbers-SEO, where people are looking for formulas and ratios and "hard numbers" for optimizing sites. IMO, SEO is an art. Just as paint-by-number pictures will never be as good as an Old Master painting, so "paint-by-numbers SEO" will never be as effective in the long run as real
site optimization — of which, effective copy written to please and persuade humans (not crafted to some artificial keyword density to allegedly satisfy search engines) is a big part.
If on the other hand you're asking how to get more such writing assignments, there are any number of online exchanges where you can sign up and bid on freelance assignments. A few that spring to mind are elance, guru.com and Craigslist. Of course, you'll also want to have your own website where you promote your own writing abilities (i.e. write your own copy for the site), which can bring you some direct inquiries as well.