This meeting is briefly described as part of Mormon’s introduction to the story of the mission of the sons of Mosiah in Alma 17:1. This flashback is not returning to “normal time” and Mormon is picking up the story where he introduced it, with the meeting of Alma and the sons of Mosiah, a meeting that was purely by chance in any world except that guided by the Lord. Of this meeting, Mormon notes:
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.
Alma meets his friends while he is on the road. This explains why he is the first to meet them, and why they will have to specifically go to the chief judge, as noted in verse 20 below. It also tells us that the entry point of the Anti-Nephi-Lehies into the land of Zarahemla is somewhere in a line from Gideon to Manti.
The small 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 a significant length of time. During that time there has been no communication with their homeland, and their journey into an area that was clearly hostile to Nephites must have been understood as extremely dangerous. Therefore there would have been great joy just to have them return, but to have them return with such success would have been even greater.