I adore my coworkers, I agree with the post you're replying to. An environment that encourages random "walk up" interruptions is a terrible environment in many ways. With an office with a door it's easy enough to shut the door to indicate "I need to be left alone now" and open the door when you're in an "it's OK to interrupt me" frame.
They did not say "encourages random walkups" - the tone and context makes it seem pretty clear that they do not want to be approachable to their coworkers.
