The "Direct Session" dimension, not to be confused with the default channel grouping or source dimension, is something different. It can tell you if a session was genuinely from its reported campaign (reported as No), or if it was simply bucketed there due to the last non-direct click attribution model (reported as Yes). I didn't mention this dimension in the article because it has some flaws which can cause brain bending complexities, plus it goes a little beyond the scope of this article, which I tried to gear more towards marketers and non-expert GA users. It's certainly an interesting one, though.
The term “organic traffic” is used for referring to the visitors that land on your website as a result of unpaid (“organic”) search results. Organic traffic is the opposite of paid traffic, which defines the visits generated by paid ads. Visitors who are considered organic find your website after using a search engine like Google or Bing, so they are not “referred” by any other website.