The "?" story is about the error handling of the ed line editor. In the event of erroneous input, it just prints "?". The original reason for this, is that Ken Thompson had better things to do than add code for nice error messages – initially the only people using it was himself and immediate colleagues who could just ask for his help in person if they got stuck. Also, the minimalist error handling was valuable in an era of extremely constrained resources (very limited memory, 300 baud modem connections, etc) – a smaller editor could be used to edit larger files, and briefer error messages made editing over slow connections faster. As soon as there was a need for a more user-friendly editor, people built new ones and left ed as it was, and those new editors soon had much better error handling.

To associate its spartan error handling with security is a bit of a retcon. Interesting analogy, but the minimalistic error handling of ed was not motivated by security concerns and had basically zero security benefits (unless you count poor usability as a form of security through obscurity.)