A very large number of people do slack off at these companies. Some engineers code only a few hours a week. They do attend meetings and such, but the actual task delivery timeline is very relaxed.
But how do you know that’s actually true? They might be just posting that to sound cool in some social app. In these companies everyone has a manager and assigned to a project. I don’t think it’s like in the tv shows or in consulting companies where you can spend months waiting for a project and actually doing nothing.