He's saying that you'd need to add other stuff to make the "good" armor much rarer, the rarity is at civ level rather (assuming that's what you're referring to) than individual level.
So for example
[ARMOR:ARMOR_ITEM] is the same as [ARMOR:ARMOR_ITEM:COMMON], which means civs of this race will usually have access to that specific armor item, but it's not 100% world to world or civ to civ of the same race.
[ARMOR:ARMOR_ITEM:UNCOMMON] means just that, it's more of a coin flip if it'll be present in a given civ of that type.
[ARMOR:ARMOR_ITEM:RARE] means that the item isn't typically found among this civ type, but might turn up once in a while for some lucky civ out of the bunch that might have genned.
[ARMOR:ARMOR_ITEM:FORCED] means that this civ will always have that armor item, no matter the world gen, no matter the civ.
To dilute the good stuff, you'd need to add additional items, so say a couple leather and cloth armor variants that count as melee armor on the [LAYER:ARMOR] at armor level 2 or 3 and is [SHAPED] so only that can be worn.
So if you have let's just say, 3 cloth armor variants and 3 leather armor variants, but still just one breastplate item available, then 6 out of 7 soldiers will have a good chance of having the crappier armors. That said, armor outfits are randomly generated for world gen troops, so you might have a force with little or no metal armor doing that, or majority metal armor.