The verb here in the original text reads in the past tense, in both manuscripts and in the early editions (although knew is miswritten as kew in 𝓞). For the first printing of the 1852 LDS edition, knew was replaced by know, perhaps unintentionally. The same change was also made in the 1874 RLDS edition, perhaps independently. The subsequent 1892 RLDS edition restored the original knew to the RLDS text, which is unusual since that edition rarely departed from the reading of its copytext, the 1874 edition.
There is a tendency in the text to change past-tense clauses to present-tense ones when the clause refers to the speaker’s knowledge, as in the following examples involving the verb know:
The same tendency has affected the past-tense supposed in nearby Alma 56:37 (“and as we supposed that …”), which was changed on three separate occasions to suppose (once by Oliver Cowdery as he took down Joseph Smith’s dictation as well as in the 1849 LDS edition and the 1858 Wright edition); see the discussion under that passage. In all of these cases, the critical text will maintain the past-tense verb forms in these clauses that refer to the knowledge of the speaker or writer.
Summary: Maintain the past-tense knew in Alma 56:43 (“and now whether they were overtaken by Antipus / we knew not”).