Here in the printer’s manuscript, Oliver Cowdery initially omitted the subordinate conjunction as, but almost immediately he supralinearly inserted the as (there is no change in the level of ink flow). The original manuscript, not extant here, most probably had the as. Elsewhere the original text has at least ten occurrences of therefore as, including a nearby one that also has a following behold-clause:
For one additional case where the as in therefore as has been deleted (in this case, when the 1830 compositor set the type), see under Helaman 7:10. Here in Alma 17:27, the critical text will accept the almost immediately corrected reading in 𝓟: “therefore as Ammon and the servants of the king were driving forth their flocks”.
Summary: Accept Oliver Cowdery’s inserted as in Alma 17:27; this virtually immediate correction in 𝓟 appears to be based on the reading of 𝓞, no longer extant here.