There has been some tendency in the middle of this passage to replace the plural they with the singular he. The last part of the verse (“and this he knew that Amalickiah would do”) suggests that the pronoun he should occur in the preceding portion of the verse. The replacement of they with he has occurred three times in the history of the text for this passage. The first time occurred when Oliver Cowdery wrote this passage down in the original manuscript. There he initially wrote that he followed by the initial w of would, but then he immediately crossed out the that he and erased the w, overwrote the w with the initial th of the that, then wrote inline the rest of the that followed by they and the remainder of the clause. In other words, the original text must have read “for he knew that they would stir up the Lamanites”.
Later the compositor for the 1841 British edition accidentally set “that he would”, but the earlier they was restored in the subsequent LDS edition (1849). In the 1852 LDS edition, the original plates continued with the they, but in the corrected plates for the second printing, the they was replaced by he. All subsequent LDS editions have continued with the singular pronoun he, but the RLDS editions have continued with the original plural pronoun they.
Despite this preference for singular reference elsewhere in this passage, there is really nothing wrong with the original they. Moroni is quite aware that both Amalickiah and his followers are fully capable of stirring up the Lamanites.
Summary: Restore the original plural pronoun they in Alma 46:30 since the immediate correction in the original manuscript clearly indicates that the original text read in the plural (“for he knew that they would stir up the Lamanites”).