Leaving aside the plausibility of a 12-year-old knowing Unix, simply knowing Unix is not enough to immediately use any application running on the system. Yes, she could probably have used vi on the security terminal. But the specialized security system would have required some learning time -- significant learning time if it were built on Unix, which has notoriously inconsistent user interface design and thus makes it harder to transfer skills from one application to the next.
This guy didn't do his research. It wasn't that specialized of a security system.