Surely the "I never tell anyone my pin" is just an admission that they do have something to hide?

The point is that there are many things that should be kept private/secret and often the need for that secrecy isn't obvious to people who have never been in particular situations. A woman trying to escape from an abusive relationship may need to keep her location secret to avoid being murdered by her ex, but your typical white male who declares "nothing to hide" may have difficulty in understanding that, whereas they may be able to grasp why their PIN should be kept secret.