for some reason the damage is calculated with pre-mitigation values
That's because it deals physical damage, if it didn't use pre mitigation values it would get checked by their armour twice, Eg. if zeds q dealt 500 damage with 50% bonus from ult against a target with 50% mitigation using mitigated values the armour would reduce to 250, then the ult would reduce to 125, then the armour would again reduce to 63. By using pre mitigated the ult reduces it to 250 then the armour reduces it to 125.
I do think it could be an interesting nerf that made it far more difficult to duel tanks and bruisers if it used mitigated values though, as it would make his ult super weak VS armour stackers.