Oliver Cowdery initially copied this passage into 𝓟 so that the first two verbs were in the past tense (“the things that they had said were true”). Almost immediately he corrected these verbs to the present tense: “the things that they have said is true”. The two supralinearly inserted present-tense verb forms were written without any change in the level of ink flow. The use of the presenttense perfect have in “they have said” agrees with the usage in the following main clause (“they have brought twenty-four plates”). Similarly, the present-tense is (in “the things … is true”) agrees in tense with the are in the last clause (“which are filled with engravings”). The present-tense usage is clearly appropriate since king Limhi is referring to the search party that returned “not many days before the coming of Ammon” (Mosiah 21:26).
Of course, the singular is in Mosiah 8:9 is nonstandard and was grammatically edited to are for the 1837 edition. Notice that Oliver Cowdery’s original past-tense were was in the plural, so if he had simply been editing the text from the past tense to the present tense, he probably would have written are (“the things that they have said are true”). The fact that he corrected were to is rather than to are is strong evidence that the original manuscript read is instead of are—and that it was indeed in the present tense. For further discussion of nonstandard cases of subject-verb agreement in the text, see under 1 Nephi 4:4; also see the complete listing under subject-verb agreement in volume 3. The critical text will restore all such cases of subject-verb disagreement whenever they are supported by the earliest textual sources.
The 1920 LDS edition restored Oliver Cowdery’s initial use of the past-tense perfect had in one of the two cases, thus creating an odd mix of the present and past tenses: “the things that they had said are true”. But this change appears to be a typo since it is not marked in the 1911 Book of Mormon used by the 1920 committee for indicating textual changes to be made in the 1920 edition. The textual changes in the 1920 edition that are not marked in the committee copy are often typos. (For a complete list and analysis, see the discussion regarding the 1920 LDS edition in volume 3.)
Summary: Maintain the original present-tense verb forms in Mosiah 8:9: “and for a testimony that the things that they have said is true they have brought twenty-four plates which are filled with engravings”; the nonstandard use of the singular is is apparently the reading of the original manuscript, no longer extant here.