While copying the text from 𝓞 into 𝓟, Oliver Cowdery accidentally omitted “the army of ”. All the printed editions lack this instance of “the army of ”. In this case, either reading will work. The critical text will restore the reading of the original manuscript.
This error is interesting in that the original “the army of ” should have been restored in the 1830 edition since we know that the 1830 signature for this part of the text, the 22nd, was proofed against 𝓞 (see the discussion under Alma 42:31). It seems that the proximity of two instances of the word army (“Moroni and his army met the army of the Lamanites”) facilitated the loss of “the army of ”, not only in copying from 𝓞 into 𝓟 but also when the 1830 signature was proofed against 𝓞.
Summary: Restore “the army of” in Alma 43:41, the reading in 𝓞: “Moroni and his army met the army of the Lamanites”.