My guess is copper. They have always been very careful to say to use coolant not water. That and it looks a copper "PD360" on their site we got:
http://www.syscooling.com/products/Radiators/54.html . The two aluminum radiator's they show selling are much smaller then the one with kit.
I could be wrong if they used a specialty one for miners, but I'm highly guessing its their standard.
Yes, you have to assume its copper or there are copper components. In several points in the listings (all changed by now) there has been a mention of copper on both the radiator and the fittings [internal coating]. You can see within the chamber of the radiator that there is definitely some form of dark orange coating, which may be brass or copper. So tldr, be safe and assume there is copper and aluminium within the same loop.