Clean in the sens: "Clean is better than an mess". The article seems to be written for non-designers who know practically nothing about design. And for them, clean is something great; it means they can effectively represent their ideas.
Initially, when I was making this site, I wanted to have an area for people to have conversations about it, in part so I could explain all these little details. There's a lot I wanted to explain that I just couldn't fit into the site because I thought it would be too tangential.