I don't think it's that shocking some adcomms might turn to a well-known board specializing in grad school applications to see if anything turns up. I've heard about applicants being googled before, too.
The example from the Linguistics sub-board just shows that the general rule of Internet-forum-use applies here too: either keep identifying details vague, or put it out there and stand by it, knowing you run the chance of being identified. It could be anyone - other classmates, relatives, professors. This board definitely has a welcoming, safe haven vibe to it, and it's wonderful, but we shouldn't lose sight of the fact that it's still a publicly accessible corner of the Internet.