Thermalright's full copper variants perform better than their aluminum versions across the stack, even under prolonged stress tests. Back in the day, the solid copper Cooltek LP53 consistently beat the pants off of Noctua's L9i, but they never updated their cooler for modern sockets. Granted, it is 42mm vs 37mm with the same fan, but the cooler design was nearly identical with a mere 2 heat pipes.@sneedster A common misconception: in the end the air is cooling, not the heatsink. Aluminium is better at dissipating heat to air than copper and the copper heatsink would just add mass that can be saturated. As soon as the copper is saturated the temps will be higher than with the Aluminium heatsink.
For bigger heatsinks the cheer size of the heatsink nullifies this effect due to the air cooling enough to prevnt saturation int he first place. But for these tiny as it can get heatsinks it matters as they saturate under load. Most likely the "adavantage" of the X36 in this test is just the fin orientation.
You can see in your linked test that there are basically no differences between the coolers - 1-2% could just be anything from measurement error, to thermal paste application, to room temperature.
From a metallurgy standpoint, what you are saying is true.... but in the SFF space, copper makes a big difference. Personally, I think it's because coolers of this size category are limited by how many heat pipes they can effectively leverage. Copper spreads the heat more effectively, making better use of the fan and surface area. I've verified this myself with an AXP-47 Full Copper.
The link @sneedster shared was a good, but not great comparison. The frequencies were not locked, so temperatures won't be that different. What is significant is that the AXP-37 was cooler at a higher core frequency. Personally, I think a solid copper Noctua L9i would be a beast of a cooler and would see more gains going copper than the AXP-37.
EDIT: All of this is relevant to the DAN A4 Nano. I want to see a cooler that destroys the L9i in this space.