Nope, it's static enmity reduction. Just like community rep saying.
So for example if the static enmity reduction is really 200 damage; Then if you're wearing 0 enmity gear:
1. If you do below 200 damage, you gain no enmity, and the targeted player will generate enmity based on whatever ranged damage you scored
2. If you do above 200 damage, then you gain (your ranged damage-200) worth of enmity, while the targeted PC generate 200 damage worth of enmity.