The elders say that God’s wrath “came” and so they are positioning it in the past as something that had already happened when they sang their song. They have witnessed the consequences of the Lamb opening the seven seals and prompting the blowing of the seven trumpets—and those actions resulted in God’s plagues coming upon the earth as described in Revelation 6:1 – 11:19. This is not to say that God’s wrath was exhausted by the time the elders sang this song, for the opening of the seals and the blowing of the trumpets describe the things that were to take place in the course of the NT dispensation (Revelation 1:1).
18 The nations raged, but your wrath came, and the time for the dead to be judged, and for rewarding your servants, the prophets and saints, and those who fear your name, both small and great, and for destroying the destroyers of the earth.”