The 1888 LDS edition accidentally omitted the word all here, perhaps because the preceding text reads without any all (“they were on the east by the entrance”). Another possibility (suggested by Don Brugger) is that the typesetter’s eye skipped from all to the visually similar first part of the following word, asleep. The word all emphasizes that the Lamanites were all asleep: no one was on guard—or if some were, they were asleep. The use of the all thus explains how Moroni could enter the city without being observed. This error was not transferred to any subsequent LDS edition because the 1888 edition never served as a copytext.
Summary: Maintain in Alma 62:21 the original all in the clause “they were all asleep”; the use of all means that no one was awake, not even the guards.