Alma 3:18

Now the Amlicites knew not that they were fulfilling the words of God, when they began to mark themselves in their foreheads; nevertheless they had come out in open rebellion against God; therefore it was expedient that the curse should fall upon them.—

The Amlicites did not know they were making God’s words come true when they began to mark themselves on their foreheads. But they had fought against God. So it was right that the curse should come on them.

The Amlicites had no idea they were fulfilling God’s prophecy when they marked their foreheads. But they had rebelled openly against God, so it made sense that the curse would fall on them.