If you re-read my earlier post (and perhaps earlier posts on other threads) then maybe you'll realise what I was saying.
But, Roman hastati, principes and triarii, liby-phoenician spearmen, phalangites and hoplites are all heavy infantry. However many of these take no more iron to equip than Iberian scutarii or thureophoroi and they are not.
It's about fighting methods, formation, drill. Like I wrote most armour wasn't iron.
Instead abstraction based on the military systems and populations. With potential for reform of those systems.
I’m going to go out on a limb here and suggest that better access to trade and resources - and the higher smithing and production expertise that accompanied this access - led to the difference in emphasis. So higher iron resource access implies higher levels of resource development (smithies, innovative smithies) etc.