As the article say, like it or not, clear expectations are set by many, if not most companies that salaries are kept private. You won't find it in the employee handbook (as it's illegal to have this "rule") but it's still clearly communicated. Most people find it in their best interest not to talk about it, again as the article points out.
Whatever the "expectation" is, people talk about their salaries, compare
them with their friends within and outside of the place where they work.