One reason why they are more knock tolerant is... less power. The heat loss is power not reaching the crankshaft. This is why you do not remove carbon from the chamber and exhaust ports: it's a thermal barrier. Back to back tests done on BBC 50 years ago: iron wins.
How does built up heat in the head get to the crankshaft? If both motors are maximized for their head material aluminum wins every time. NOBODY fast runs iron heads unless rules of their class dictate they must.
