Here Oliver Cowdery initially wrote “these mysteries art”. Later, perhaps when he read the text back to Joseph Smith, he crossed out the art and supralinearly wrote are (the correction was made with somewhat heavier ink flow). As discussed under Mosiah 2:24, there is some evidence that once in a while Oliver accidentally crossed the final e of are, giving art. This example of art here in Alma 37:11 appears to be an instance of this error, one that Oliver corrected. There are, on the other hand, a few instances of art (chiefly in relative clauses) that appear to be intended, although nonstandard for the biblical style deriving from Early Modern English (for discussion of this possibility of actual art, see under Alma 32:15). But the use of initial art here in Alma 37:11 appears to be a scribal slip on Oliver’s part. There are no other examples in the text where a plural subject takes an immediately adjacent art as its verb.
Summary: Accept in Alma 37:11 Oliver Cowdery’s correction in 𝓞 of his initial art to are, the probable reading of the original text: “these mysteries are not yet fully made known unto me”.