Gurglamei, part of your question can really be restated as: "When a slider in a raw converter has the value of 'zero' -- what does that really mean?" Some raw converters will certainly do noise reduction even when the noise reduction slider is set to zero. In other words, "zero" doesn't necessarily mean "completely disabled."
It also depends on your definition of noise reduction. At a technical level, as explained above, the demosaic step involves interpolation. A common approach (though not required) among demosaic algorithms is that the original pixel values are preserved, unmodified. For example, the original green pixel values at green pixel locations are maintained, while the demosaic needs to synthesize green values as the red & blue pixel locations. That synthesis usually occurs by some form of interpolation, which almost always reduces noise. That is, when using standard interpolation methods such as bicubic, bilinear, etc. the variance of the interpolated green pixels is less than the variance of the original green pixels. Hence, taken as a whole, the variance of all of the green pixels (both original + interpolated) has decreased. Thus, one can argue that ALL such demosaic methods (which basically includes all of the commercial ones) include some amount of noise reduction during the demosaic.
