Aluminum is a good heat conductor for a solid material, but it's only a "heat sink" when the geometry is such that the aluminum has a ton of cool surface area per heated surface area (IE a CPU heat sink). If it's just a sheet of aluminum it's a 1:1 ratio - I'm pretty sure it's a heat shield. Not a heat sink. The geometry makes the difference. Either way, I don't think it's worth worrying about here, but just figured I'd throw that into the conversation.
Which good conductor is not a solid material?
Aluminum is a good conductor because of it's atomic structure, that has nothing to do with it's geometry. Fins certainly make it much more efficient but it's not like it stops transferring heat from hot side to cold side just because the it doesn't have more surface area on one side than the other. Reference fanless laptops or fanless amplifiers built with aluminum cases. Aluminum is certainly reflecting some radiant heat back into the hot space, but the conduction and convection shedding the heat should win out.