In the printer’s manuscript, Oliver Cowdery initially wrote “for I say unto you that there are many which have fallen by the sword”; then virtually immediately he crossed out the that. The original manuscript is not extant here, but the text fits best (although still imperfectly) within the lacuna if there was no that in 𝓞. Ultimately, there would have been no motivation to edit the text here since both readings are possible (see the discussion under 2 Nephi 30:2 for that after “I say unto you”). Earlier in this verse, we have an example without the that: “I say unto you : if ye have supposed this / ye have supposed in vain” (Alma 60:12).
Summary: Follow in Alma 60:12 the corrected reading in the printer’s manuscript, namely, the removal of the that after “I say unto you” (𝓞 is not extant here).