Here we have another virtually immediate correction by Oliver Cowdery in 𝓟. He initially wrote through, but then he added the out supralinearly and without any change in the level of ink flow. Although through is possible here, the text prefers throughout in the phrase “through(out) the land”. The critical text will here accept the corrected reading in 𝓟. For further discussion regarding through and throughout, see under Mosiah 29:1.
Summary: Accept in Alma 16:1 throughout, Oliver Cowdery’s virtually immediate correction in 𝓟.