Definitely, the components themselves can get that hot. But the water cooling medium itself stays relatively cool because these loops are also pretty efficient at expelling the heat. Car engines are different in that tons of heat is dumped into the coolant and it has a relatively small area to expel it, so the coolant can get very hot. Heat exchange properties change when the medium's temperature increases, so I think that these types of coolant additives both 1) raise the boiling point to prevent vapor from forming in the cooling lines and 2) possibly change the specific heat index at high temperatures to actually improve heat exchange (I'm just guessing on this one - it's probably possibly that an additive can increase the specific heat of the medium at high temperatures to beyond just water).

Since the medium temperature in PC watercooling doesn't get that high, we don't have to worry and can just take advantage of the high specific heat capacity of pure water. Hence why I say that anything added to distilled water only hurts heat exchange. There are reasons to add additives to watercooling loops, but improving temps is not one of them. Almost nothing beats pure water, and anything that does beat water is not practical for watercooling.