In addition to what others have said, please note that while your information may be hidden, most of the people you are matched with do not have the same setting. Their ending report and the information the API makes available includes your information, even though you may have your own information hidden directly. The third-party sites do not need to access your information directly in order to use it for data collection and calculation; you have up to 11 other people (possibly more, if someone left and another joined) that shows the full information for everyone involved at the end of an activity. You can hide your activity directly, but you can't hide theirs.