This is not a figure of speech. It is not a reference to scripture. It is very certainly a direct reference to both the angelic communication to Alma the Younger and to Amulek (see Alma 11:31).
In using the term "them that are scattered abroad," Alma continues the practice that Nephi began when he first assumed that his people were the scriptural referents of those who were on the Isles of the Sea. While we don't have many direct evidences of Book of Mormon prophets reading the scriptures that were in their possession, this is another clue that Alma had read extensively in the Nephite scriptures.
The story of Melchizedek reminds us that he had read the brass plates. Here, the likelihood is that he has read Nephi. Of course, this is not at all surprising. We may expect that all of the prophets read at least some of the scriptures of the past. Alma the Younger's conversion was so dramatic that we may expect that he was thirsty for information, and read all of the records.