The only requirement to be an atheist is to not believe in a deity. There's no such thing as a "hardcore atheist", because there's no central dogma to be hardcore about. Many atheists do follow some sort of non-theistic belief system, such as Buddhism or Daoism.

You're thinking of the people who are what I'd call "anti-theistic", that is, explicitly opposed to the concept of deities, religion, or mysticism in general.