The adverbial phrase that day was initially omitted by Oliver Cowdery as he took down Joseph Smith’s dictation. His correction in 𝓞 appears to have been virtually immediate since there is no change in the level of ink flow for the supralinear that day. There would have been no motivation for Oliver to have emended the text here since the sentence reads perfectly well without specifying that day (from the larger context it is obvious that the conversion of these Lamanites took place on the same day).
Summary: Maintain in Alma 24:26 the adverbial phrase that day, the corrected reading in 𝓞.