That's BS (and a very common myth in heatsink discussion), no metal can dissipate heat better or absorb heat better - it's all the specific heat, and since copper's specific heat is higher than aluminum it will be better at absorption and dissipation - better at transferring heat. The main reason aluminum is used for the fins is that all copper fins (see TRUE copper and Ninja copper) are way too heavy.
Before this turns into an argument, I would like to point out that my original post was to add an extra piece of information and mainly to extol the quality and design of the Thermalright cooler. All points are well taken.