The original manuscript is not extant here, but spacing between extant fragments of 𝓞 suggests that there was room for the longer these. Oliver Cowdery, when copying to the printer’s manuscript, initially wrote the, then inserted the se inline. His addition of se may actually be purely scribal since the the occurs at the end of the line; that is, he may have decided to insert the se at the end of the line rather than write it at the beginning of the next line (thus he would have avoided hyphenating the word). There is no change in the level of ink flow, which suggests that this correction was virtually immediate. (For a list of cases where the scribes tended to write the instead of these, see under Jacob 1:1.)
The use of these also makes sense. The directors seem to have been at hand as Alma conversed with Helaman (see the immediately previous discussion regarding the dominant use of these throughout this part of the text). The definite article the would work only if Alma had already mentioned the directors, but he had not. Further support for these directors is found several verses later where the text once more uses these: “and now my son these directors were prepared that the word of God might be fulfilled” (Alma 37:24).
Summary: Maintain in Alma 37:21 the use of the demonstrative these (the corrected reading in 𝓟) to refer to the directors: “yea and that ye preserve these directors”.