Not only did I read the text, but my long technical background conditioned me to reject ambiguity in critical processes, and NEVER to rely on inferences.
That Tesla "added" something does not clearly report that it is a feature on just the most recent models sold, or they recalled all previously sold cars. See the problem?
Since the article didn't include anything to the contrary, one must assume that ALL cars sold are prone to the same failure, upgraded or not.
In addition, the "added" feature could well, have been powered by a small additional rechargeable battery, which could monitor anything for weeks if not months. Monitoring would not need to be continuous. Unless the main battery voltage drops like a rock off a cliff, brief ( less than one minute) monitoring every 6 or 12 hours may suffice.
You’re obviously smarter than the Tesla boys and girls, they pay big bucks, and are located in beautiful Palo Alto, Cal.
I nominate you to go save them!
http://www.teslamotors.com/about/careers