It's true that it does do so, but that's not it's purpose in a water cooled unit. Any circulating fluid which is cooler than combustion will perform some cooling duties, but in the case of a water cooled turbo, like the CT26, or my Turbonetics, or the OP's ATS, I would not expect that it would last long at all with no water going through the unit, same as the oil will not work to keep your engine from overheating in the absence of coolant. As I stated earlier, it MIGHT be able to survive if you were to pump oil through the water passages instead of coolant, assuming that the seals will stand up to oil, but I wouldn't make any bets as to the longevity of such a setup.
For all practical intents and purposes, there isn't enough volume of oil present in a water cooled turbo at any point in time to provide anywhere near adequate cooling, and as such, I felt it better to present the lubrication system as being completely separated from the cooling system.