Nope, not going to work.
Unless you live in a freezer... The water in the res needs to be much cooler then for your CPU and it's a much larger mass. A CPU cooler works by exchanging the heat with ambient air. In a chiller its exchanged with much cooler things. A cooler will allow you to get a 'colder then the room' temp. With a CPU type cooler you can only get it has cold as the room and you need a lot of flow over a large heat sink or a very cold area to get that to work. Works for CPUs cause they are just keeping it cool as the room or a little over and a very small mass compared to a tank of water.
My .02..
I do not think he is talking about using a cpu block, but rather the res water going straight through the radiators. You aare correct in what you say, but you do not know the size or number of, or capability of the radiators in question, as that information has not been divulged, nor the size of the reservoir to be chilled down, it could just be for a tiny waterfarm. If it manages to knock a couple of degrees off the temp, then that could make all the difference for someones grow.
I am on your side though, i don't think it's really a feasible thing to do, and if the watercooling equipment is of a high quality then it could just get sold to fund a chiller. But done right it could work in some manner or other.
For reference, i use thermochill pa120.3's, and with the right fans, those things can remove a LOT of heat. But they are very expensive and yeah, you risk damaging the radiator and the pump.
