With careful planning and by following the commander Gideon’s instinct for strategy, a point of weakness was detected in the Lamanite guards watching over the city of Nephi (22:6). Flocks and herds were driven out into the wilderness (22:6), a substantial tribute of wine was delivered to the guards (22:10), and when the guards were drunk in the deep of the night, the people left through a back pass in a back wall on the back side of the city (22:6), passing on the left of the guard’s camp (22:7). They managed to take with them their women and children, flocks and herds, their gold, silver, and precious things (22:12).
Nothing is said about this being a miraculous escape—although their escape was amazing and divine forces may have helped to keep the guards intoxicated and unaware. The plan succeeded due to careful observations and planning, proper organization, detailed timing and maneuvering, and complete cooperation from everyone involved. Although this was an unusual escape, the total population was probably not very large at that point. By comparison, the whole group of Alma’s people totaled 450 people (18:35). And the Lamanites made only a small effort to pursue them. After all the trouble caused for the local Lamanites during the time of Zeniff, Noah, and Limhi, they were probably not terribly disappointed to see the Nephites gone.
Book of Mormon Central, "Why Did King Limhi Think Gideon’s Escape Plan Would Work? (Mosiah 22:6)," KnoWhy 100 (May 16, 2016).