When you increase the SuperSampling, a more accurate geometry render will be done to determine which particles need to be drawn to the “OccludedLayer” render element. You should definitely see more of a pronounced holdout border in your beauty pass because more particles are drawn to the background layer (ie. the “OccludedLayer” render element). When you go to composite you images you want the Krakatoa beauty pass on top of the geometry on top of the Krakatoa “OccludedLayer” element. This way your particles are correctly drawn in front of and behind the geometry.
Anti-aliasing of matte objects is actually the exact same process as how you would deal with partially transparent geometry, hence the front and back images.