Let me explain the analogy to you. In both scenarios you're in a public place communicating information to an intended audience. There is a good chance that some people outside of your intended audience will also get this information. You're accepting the risk that your coworkers may be on the site/at the next table and receive this information but if they aren't then they shouldn't have access to it.
In your first point you mention adjusting volume to restrict communication to your desired audience. OKCupid has privacy settings. Users adjusting their privacy settings would be analogous with adjusting volume in the restaurant. The people collecting data in this story created a fake account and used it to bypass the privacy settings. For your second point, it would be more like if you filled out a restaurant survey and saw your answers on an unrelated website like Reddit the next day.