Here one might consider the occurrence of the subordinate conjunction that after the conjunction for to be an error, perhaps the result of the four surrounding occurrences of that in the passage. Nonetheless, we can find support for for that. In this case, the conjunction for is acting like because and since, and in the original text of the Book of Mormon we regularly have that following these two conjunctions (see the discussion under 1 Nephi 1:14 for because that and under 1 Nephi 22:5 for since that). The expression for that parallels this usage, especially since here for has the meaning ‘because, since’.
Although there are no other examples of this use of for that in the Book of Mormon text, there are a number of examples in the King James Bible, as in the following:
In a modern English translation such as the Revised Standard Version (1952), these two instances of for that are translated as because (and, of course, without the archaic that following the conjunction because). For further discussion of the archaic for that, see definition 21 under the preposition and conjunction for in the Oxford English Dictionary. The OED explains that for that with the meaning ‘because’ dates from the 13th century. In addition, the King James examples show the continuing use of for that up into the 17th century. For further discussion of for that as well as because that and since that, see under subordinate conjunctions in volume 3. The critical text will therefore maintain this archaic use of for that despite its uniqueness in the text.
Summary: Maintain in Alma 21:21 the unique occurrence of the archaic for that, which has the meaning ‘because, since’; such usage was fairly common in Early Modern English.