As a trained electronics and missile guidance technician, I’m not sure I buy into that theory. Maybe some of the later “self correcting” models, but the ones I used contained no electronics at all...purely mechanical.
But for arguments sake, it would be a chore to “hack” and office with 20 or 30 Selectrics going at the same time.
Either way, it would be a hellava lot more secure than wifi’s, the internet, etc, and the NSA would have set up shop outside thousands of offices to even detect such a minute signal.
I cannot give you the specifics of the method used, but it was disclosed by James Bamford in the “The Puzzle Palace” that this was one of the NSA’s many successes.