Malachi wrote after Lehi led his family out of Jerusalem, so the Nephites could not have had these texts. When they heard them, they would have heard them for the first time. However, to have the impact necessary, the Savior must have introduced them in some way as scripture. Perhaps to create anticipation he announced that they were scripture, and did not explain until afterward that they were scriptures that they did not have. Given that the Savior expected that the people would recognize passages from Isaiah, he would assume that they had at least some recognition of the major received scriptures, and he would have know that they would listen to Malachi intently, trying to place it in their memory of scriptures they had heard.