Ammon tells him about King Mosiah.
12 And I say unto thee again: Knowest thou of any one that can translate? For I am desirous that these records should be translated into our language; for, perhaps, they will give us a knowledge of a remnant of the people who have been destroyed, from whence these records came; or, perhaps, they will give us a knowledge of this very people who have been destroyed; and I am desirous to know the cause of their destruction.
Then, we get a long side story about three generations of Zeniff's people in the land of Nephi, which includes Mosiah 9-21:27. After that long side story, Mormon resumes the narrative right where he left it off:
28 And now Limhi was again filled with joy on learning from the mouth of Ammon that king Mosiah had a gift from God, whereby he could interpret such engravings; yea, and Ammon also did rejoice.
Note that the end of the side story happens without any discrete indicator in the text, but once the side story is over, the text seamlessly resumes right where it left off.
Of course, this subtle detail is very easy to explain if you believe Mormon authored the text over a long period of time with copious reference materials at his disposal--he simply drafted the side story and entered it into the narrative at an appropriate time, then resumed his main narrative once the side story ended. If you believe Joseph Smith made up the text, you have a few things to explain.
Hat tip: Grant Hardy.