
The tier 7 buff to Horrors doesn't buff their ranged capabilities or even their melee ones. It only provides Speed, Leadership and Missile Resist that are a much lower priority for this type of units. This especially stings considering nothing in Tzeentch's tech tree buffs Horror's ranged prowess either (other than extra ammo)

The tier 7 buff to flamers doesn't buff their ranged capabilities at all. It only buffs their Spell Resist, Leadership and melee Weapon Strength, which are again, extremely low priority for this unit type that is horrible in melee anyway.
Lastly, None of Tzeentch's red line skills buff Chosen of Tzeentch.