The name Antionum was here misspelled as Antionam in 𝓟 and in the 1830 edition. The correct Antionum was restored in the 1837 edition. The name occurs five times in the text; the name is fully extant in 𝓞 for three of the occurrences and reads Antionum (including here in Alma 43:5), while in 𝓟 the name is always spelled Antionum except here in Alma 43:5. The critical text will accept Antionum as the correct spelling. Similarly, the name Cumorah was misspelled in the earliest extant sources as Camorah (once by scribe 2 in 𝓟 and always in the 1830 edition); see the discussion under Mormon 6:2. As exemplified by the spellings for the name Zoram, Oliver Cowdery’s u’s and a’s often looked alike (see the discussion under 1 Nephi 4:35).
Summary: In accord with the earliest textual sources, maintain the spelling Antionum.