You (and many others) confused correlation for causation.
Removing the lead from the fuel meant that the lead was no longer in the fuel. That caused an increase in the probabilities of detonation in engines that were not designed to run on unleaded fuel. Detonation causes a disruption of the boundary layer inside cylinders that usually helps to insulate them from being exposed to all of the heat generated by the combustion event. That’s why ‘heavy’ detonation causes an increase in CHT (and is what is causing the problem in mustering chopper engines).
I think you’ll find that what actually caused the rapid valve seat recession in car engines when lead was removed from MOGAS was detonation/high CHT.