Not true. The camera lens cover slider is not a magnetic system, it's optical. Just pop the back off an you'll see the cover is blue, with a half-white/half-black bar. That bar lines up with an optical sensor that "reads" the light bounce back for the cover to determine if it's open or closed.

The rail switch for the left rail does seem to be close to it, so that may be throwing if you put the magnet near the camera. But the camera doesn't use a magnetic sensor to sense the lens cover. If it did, a magnet there would turn it on and start the camera software. I wonder how much a 3 hour video of the inside of your case would wear down the battery?