low ISO =/= low noise. You seem to have a lot of misconceptions about what causes noise, what clipped out means, etc. That seems to be where all your confusion is coming from. The statement "That's bogus because it came out of the camera clean because it was taken at ISO 100" doesn't make any sense at all. What do you even mean by "came out of the camera clean"? Your camera takes clean photos at ISO 100 WHEN PROPERLY EXPOSED. High ISO can amplify noise, but that doesn't mean that low ISO's don't also amplify noise, they're just less sensitive to it, so it would take longer exposure to the noise for it to be noticeable with a low ISO. ISO doesn't make noise, cameras and environment make noise. ISOs, in digital cameras at least, AMPLIFY noise. WHen you brighten a picture with software, you're essentially doing the same thing, you're amplifying the lowest light signals from the sensor to be brighter. So, if your camera produced a lot of noise, it doesn't matter what ISO you shot it at, the noise your camera created will be amplified (along with any environmental noise). Again, ISO doesn't create noise, so saying your camera takes clean shots at ISO 100 is completely meaningless. All that matters is how much that noise was amplified, and how good the programs that reduced it were.
This is the whole point that everybody has ben trying to make. If you have low enough noise, you can underexpose almost much as you want to. There is no such thing as 'clipping' on the left side of the histogram. Sure, with cameras that create lots of noise the left side can become pretty unusable pretty quickly. And some older sensors don't pick up low levels of light very well, especially in short exposures, meaning that they can run into noise problems really fast from environmental noise. But as long as the sensor registered photons, you don't lose that information, and it's certainly not like you lose information on the right side, where it just disappears, as the sensor cannot physically record above a certain level.
Clipping has to do with overloading the sensor on the right side of the histogram. The left side being unusable has to do with the signal to noise ratio.