Mormon briefly describes this meeting in Alma 17:1: “And now it came to pass that as Alma was journeying from the land of Gideon southward, away to the land of Manti, behold, to his astonishment, he met with the sons of Mosiah journeying towards the land of Zarahemla.” This flashback has now brought the action to that previous point, with the meeting of Alma and the sons of Mosiah. Only Yahweh could have guided such a meeting.
Alma was on another missionary journey to Manti, which he interrupted to return to Zarahemla with Mosiah’s sons to discuss the situation with the chief judge (v. 20). This detail also confirms that the Anti-Nephi-Lehies enter the land of Zarahemla somewhere on a line between Gideon and Manti.
The phrase at the end of verse 16, “this was a joyful meeting” must be one of the greater understatements of scripture. The sons of Mosiah have been away from their homeland for perhaps thirteen years. During that time, they have had no communication with their family and homeland, They were going into an area clearly hostile to Nephites. It would not have been pessimistic for their friends and family to wonder if they were dead. To have them return—all of them—would have been a great joy in itself; but the joy expanded at having them return with such missionary success.