The terms are confusing.

In electronics, a "heat sink" is a device that attaches directly to a component with a temperature problem. Heat transfers from the hot component to the sink by conduction (based on the shared surface area and thermal characteristics). The sink has fins from which the temperature is lost by radiation. There is no liquid exchange.

The devices sold as heat sinks appear to be remote radiators which receive hot ATF from (and return to) the transmission, and cool mostly by convection (air movement due to vehicle speed) and less by radiation. Since many do not have fans and are not located in an air path with a pressure differential, they are less effective.
Some locations may place the sink in hot air from the engine bay, exhaust, water radiator, etc. and do very little. If you're going to use one, I suggest placing a temperature probe in a few places and compare.