The twenty-four plates containing the book of Ether described the history and the secret abominations of the Jaredite people, secrets which brought about their destruction. Therefore, Alma tells Helaman to keep the information secret. Since the original plates could only be read by means of the interpreters, Helaman is to keep the plates and the interpreters safe. However, Mosiah had already made a translation, and while that translation is not mentioned specifically, it, too, would have had to have been kept from any further dissemination among the Nephites. Even when Mormon told the story from his perspective nearly five hundred years later, he declined to include that information during the story when he described Mosiah reading it to the people (see Mosiah 28:19).
When Alma says that “ye retain all their oaths” (verse 27), the intent of the word retain is to hold back, a usage that we see in describing a retaining wall.