I'm pretty sure the watch battery idea was conjecture. I recall no such statement from TV, and have been looking for clues. The second part (about keeping it simple) is I think exactly right (although I never 'heard' them put it that way), which I think lets a lot of things out. Including LEDs. My money's on a bulb, and a fairly interesting one at that.
How to make the 'reverse logic' version (light on when power is off, off when it's on) is easy, the light switch in my bedroom does that. Not very useful to us here.
I still don't understand how it can work as described and am looking forward to finding out about it when the time comes. While I might well be embarrassed by a 'oh, wow, of course!' moment, I don't plan to be shy about what I understand is going on....once I understand it.
OF