I have no idea what video you are referencing.

The problem with cast iron is one spot can be significantly hotter than another spot, a ridge in the combustion chamber for example, you may have a 200 degree coolant temp but a hot spot that is 300 degrees because the heat is not transfered away very fast and the rest of the iron just slightly above the coolant temp perhaps 220 degrees. Aluminum on the other hand can also have the 200 degree coolant temp but the same thickness aluminum as the steel was but that one spot won't get as hot, the heat can move away from that area into the surrounding area quicker so you have a more consistent temp of the aluminum in the actual chamber, the temp in the same hot spot might get to 250 and the rest of the aluminum perhaps running 225 degrees so the average heat of the chamber is about equal but there are not as dramatic of hot and cool parts. That is why I brought up about the heating broken bolts, the aluminum head needs more heat applied to that spot to get it hot enough to remove the bolt and that entire aluminum head will get warm and the iron head will not but look out because that one spot you heated will burn your finger for several minutes. There is not a significant heat transfer from just one split second of heat but after thousands and thousands of those flashes of heat there are going to be hotter areas in the iron head than an identical aluminum head would have achieved.

Maybe a better analogy for you would be a well insulated and sealed up house with the thermostat set to 70, the entire house is close to that 70 degrees but then you got a drafty old house where you may achive 70 right there at the thermostat but there are much hotter and colder places in the house.

I don't understand how someone can claim the heat transfer is enough to make a difference in efficiency because less heat is absorbed by the iron, but on the other hand not understand how in one isolated part of the chambers temperature enough to make a hot spot that can start preignition when the fuel and air mix is squeezed to 250PSI against one spot that is already hot because it can't transfer it to the coolant fast enough...