I think the person speaking the prophecy (Malbeth the Seer right?) had seen that this would happen in the future (why else would he be a seer?). He had seen that it would happen this way and not another, so there was no other possibility for the witch-king to die. Of course it would have been possible for anyone else to kill the witch-king, but since history was going to take this course, it was to go like this and not in any other way. I don't mean to say that because Malbeth said this, it was predetermined; he had just seen what would be the course of history.

