I think a better expression would be "to regulate fermentation." Kimchi lasts quite a bit longer when stored in kimchi fridge, because (I assume) fermentation

I think a better expression would be "to regulate fermentation." Kimchi lasts quite a bit longer when stored in kimchi fridge, because (I assume) fermentation almost stops, due to consistently low temperature. But obviously you don't want it to freeze (frozen kimchi tastes absolutely terrible), so the fridge must stay just above the freezing temperature, for it to work well. And in my experience, every kimchi eventually goes sour in regular fridge (or even in a special kimchi fridge, if you keep it long enough), so I don't think fermentation completely stops at those temperatures.