I think the poster isn’t talking about the remote server seeing client MACs (because, as you say) but, rather, about the way mobile OSes use nearby wifi access

I think the poster isn’t talking about the remote server seeing client MACs (because, as you say) but, rather, about the way mobile OSes use nearby wifi access point SSIDs to provide GPS-less geolocation. Samy Kamkar raised my awareness of this way back in 2010 or so, and this article about his DefCon talk provides a good quick explanation: https://www.bbc.com/news/technology-10850875 https://www.bbc.com/news/technology-10850875.