The short answer is, you're fine at ISO 100.
The longer answer starts with post #3 in the thread you quoted:
Originally posted by Marc Sabatella A sensor has one and only one "native" ISO - that is, it only actually collects photon at one given rate. All higher ISO's above that are created by amplifying the results collected at the native ISO. The more amplification, the more noise. A lower native ISO means more amplification is required to get to, say, ISO 1600. Meaning it's likely to be noisier at ISO 1600 than an otherwise-similar sensor with a higher native ISO.
And there's no good way to "de-amplify" the results collected at native ISO in order to simulate lower ISO. A shutter speed slow enough to get a good exposure at ISO 25 would clip at the native ISO of 100 or 200. I gather some cameras have implemented some sort of magic to get around this, but it's probably more work than it's worth given that a ND filter solves the problem nicely.
The Nikon D300s mentioned in the second link does have that magic trick. Here's a qoute from the dpreview.com review:
"The base sensitivity of the sensor used in the D300S is ISO200. There is also a Lo 1.0 mode that attempts to mimic ISO 100 but it's effectively just ISO 200 over-exposed by a stop. The result is that the sensor becomes saturated and clips to white quite easily, limiting that mode's dynamic range. For most applications, you'd be better off buying a neutral density filter if you need slower shutter speeds than ISO 200 will allow."
Your camera does not do this, so ISO 100 really is ISO 100. Both cameras also have a confusingly similar feature to cram more information into JPEG files. Pentax calls it D-Range and Nikon, Active D-lighting. Some magic is applied to JPEG processing with that setting too. It has no effect if you shoot RAW.
Although the highly technical camera review sites can overemphasize small number differences into giant problems, it's worth reading the review for details like this.
Oddly enough, I briefly met both these guys at the same moment.