It's Lettvin's hack scaled to the point of meaninglessness. The whole point was to get them physically in his office and talking to him, and I'm pretty sure he was only interested in students to begin with. If he wanted to talk to everyone who had ever read the book, he may as well just put an ad somewhere.
Maybe I wasn't very clear: the post made it sound like the point of the "hack" was to identify and recruit talent (i.e. if you were interested in the book you were probably talented). Dealing with physical books, on campus, obviously limits the pool of potential talent. Something like Kindle's notes (or its next, more refined, version) could not only identify who might be talented (i.e. who had read the book), but the notes/bookmarks/highlights could also provide insight into just how talented someone was, whether that person was on MIT's campus or was some kid living in a village in India.
