The subject this seems to have been missed when Oliver Cowdery initially wrote down the text here in the original manuscript. Somewhat later, perhaps when reading the passage back to Joseph Smith, Oliver noticed the missing subject and inserted it supralinearly and with heavier ink flow. Probably at the same time, he also crossed out the initial k in his original spelling, kneeds, of the word needs. This nearby change is definitely due to editing, and one wonders if the inserted this might also be due to editing.
One possibility is that the immediately preceding word that is actually not a subordinate conjunction but a pronominal subject. Under this interpretation, the subordinate conjunction that would be lacking. But this goes against the rest of the usage in the text. Immediately following the verbal phrase “must needs be”, the Book of Mormon text otherwise has the subordinate conjunction that (14 times); in other words, there are no examples of this phraseology where the that is lacking. In all probability, the that in the original manuscript stands for the subordinate conjunction that, not a subject that. Also note the use in Alma 32:28 of that in the following conjoined that-clause: “or that the word is good”.
It is possible here in Alma 32:28 that a different pronominal subject could have been inserted, such as it: “it must needs be that it is a good seed”. In nearby sentences, either it or this can be used as a subject pronoun to refer to “a good seed”:
Since either it or this will work here in Alma 32:28, the safest solution is to follow the corrected reading in 𝓞 for this second occurrence of “a good seed”, namely, “it must needs be that this is a good seed”.
Summary: Accept in Alma 32:28 Oliver Cowdery’s inserted this in 𝓞 as the probable reading of the original text and a correction in accord with what Joseph Smith originally dictated.