Minecraft wiki entry for Splash_Potion, associated text: If an enderman is targeted directly by a splash potion, it teleports away before the potion can affect it. However, if an enderman is caught in the splash radius of a potion not aimed at it, it takes damage from the potion.
If killed by a splash potion of harming, the death message would be: <player> was killed by <attacker> using magic