In this passage Oliver Cowdery, it would appear, supralinearly inserted in light pencil the subject pronoun he. As discussed under Alma 17:8 and Alma 17:18, penciled-in changes in 𝓟 were probably made in the print shop. The two changes in Alma 17 were unnecessary and will be removed from the critical text. Here in Alma 18:2, without the inserted he, 𝓟 most naturally reads as if the preceding subject pronoun they is the intended subject (“and when they had all testified to the things which they had seen and had learned of the faithfulness of Ammon”), but quite obviously the referent should be king Lamoni, who has “learned of the faithfulness of Ammon in preserving his ﬂocks”. It is possible that the original manuscript had the subject pronoun he and that it was accidentally lost while copying from 𝓞 into 𝓟. Or perhaps the he was lost during the dictation of 𝓞. Other possible omitted subjects would have been longer (such as the king or king Lamoni ). However, the most probable subject would have been the shortest candidate (namely, he). Elsewhere, there is considerable evidence that the subject pronoun he was sometimes omitted. See, for instance, the discussion and examples listed under Jacob 5:1–2.
Summary: Although Oliver Cowdery’s secondary he in Alma 18:2 was probably inserted in the print shop and without reference to 𝓞, the original text most probably had the he.