Here we have an example of a demonstrative followed by a possessive pronoun (namely, “this their”) that was removed from the text. In this instance, the 1840 edition omitted the their, probably accidentally. The their was restored to the RLDS text in 1908. Elsewhere in the text there are three other occurrences of “this their”, none of which have been removed from the text:
The example from Helaman 4:13 is similar to this one in Alma 27:23 (both have the adjective great before the noun).
The critical text will maintain all original instances of the demonstrative followed by a possessive pronoun, such as “this their” here in Alma 27:23. For a nearby example involving “these our”, see under Alma 26:14 (in that case, the possessive pronoun our was removed rather than the demonstrative these).
Summary: Maintain the combined demonstrative and possessive pronoun “this their” in Alma 27:23 (and elsewhere in the text).