That is exactly what a heatsink is. You are using your personal experience with computers. Which has led to the wrong definition of a heatsink, not the other way around. The heatsink part (or thermal reservoir of a heat transfer system) of a GPU, a CPU, etc, is very good at absorbing heat. More than the chip it is attached to. This is because a heatsink is defined by their relative thermal capacity. It gets very hot from the chip, then a dissipator releases the heat to a fluid medium (air or water) that is, hopefully, an even better heatsink.

All heatsinks work the same way, they get hotter through various means of thermal transfer. Conductive, radiation or convection. Because they can take a lot of energy before getting hot. These properties are manipulated to change the temperature of things, but the intended (or resulting) heat exchange does not define the heatsink.

Specifically in this case, it is the water that is turning into a radiation heat reservoir by the effect the paint has on sunlight. It absorbs rather than disperse the light. But paint and concrete are a shit thermal reservoir in comparison to water, which is excellent. So, conductively, a ton of heat gets absorbed by the water, making it hotter and therefore a good breeding ground for algae.