Oliver Cowdery initially wrote them instead of the Lamanites here in 𝓟. His correction was immediate since he corrected the them by erasing the m and then writing inline Lamanites. Undoubtedly the original manuscript (and the original text) read “Amulon and his brethren did join the Lamanites”. Oliver made a similar error in 𝓟 for a nearby passage:
Summary: Retain in Mosiah 23:35 Oliver Cowdery’s immediate correction in 𝓟: “Amulon and his brethren did join the Lamanites”.