Zilean Ultimate Not Ignored By Fountain Laser

I was playing an ARAM and at the end of the game the enemy Zilean successfully revived himself with his ultimate after taking lethal damage from our fountain laser. It is my understanding that the damage dealt by the fountain is intended to ignore all immunity and revival effects, including Chronoshift. It is worth noting that the game had not ended when Zilean took the lethal damage and his ultimate was triggered.