Your mixin of Zombified Piglin doesn't implement NeutralMob.
This may be why they are always hostile.

https://mappings.xhyrom.dev/1.20.1/net/minecraft/world/entity/monster/zombifiedpiglin

@SmileycorpMC Fixed the hostile zombie piglin bug and made a PR. Pinging just in case.

Removing this mixin made the piglins no longer hostile, so this is my best guess on what is wrong with this.

Adding
import net.minecraft.world.entity.NeutralMob;
to src/main/java/net/smileycorp/hordes/mixin/MixinZombifiedPiglin.java

and changing public abstract class MixinZombifiedPiglin extends Zombie {
to public abstract class MixinZombifiedPiglin extends Zombie implements NeutralMob{

Seems to have successfully allowed the config option to control their aggression level, but only if the config infectionEntitiesAggroConversion is also off.

Managed to fix that other config too.

And I made a PR for the 1.20 branch.
#214

Fixed in dbe4d41