Yes, but no. Iron is still a good conductor of heat. It is just slower than aluminum and copper, so it is a poor material for making small materially efficient heat sinks. But it is perfectly fine for making large heavy heat sinks.

The reason it is said iron retains heat in cooking is because it has a high thermal mass (this is mainly because cast iron is so dense and non-ductile(so it has to be made thicker to not break)). It takes a lot of energy transfer to cool down a heavy cast iron pan. Conversely, it takes a lot of energy transfer to heat up a heavy cast iron pan. As long as heat-in<heat-out it will work as a cooler.