In his editing of 𝓟 for the 1837 edition, Joseph Smith here revised “as great as them all” to read “as great as all of them”. This change was never implemented in the 1837 edition. A similar occurrence of this expression, found earlier in this chapter, was left unchanged in 𝓟: “and a limnah of gold was the value of them all” (Alma 11:10).
Elsewhere in the text, the phrase them all is fairly common, with at least 11 occurrences (depending on how them all is parsed). On the other hand, there is no occurrence in the original text of all of them that is equivalent to them all. The only instances of all of them are subject forms, not object forms (thus disallowing substitution of them all ):
The first two of these are quotations from Isaiah and follow the language of the King James Bible.
Beginning with the 1920 edition, the LDS text has had one instance of all of them acting as an object form, but in the original text this read as every one of them:
Based on the overwhelming use of them all in object position elsewhere in the Book of Mormon text, a more consistent emendation for Alma 28:5 would have been to change every one of them to them all (“and thus the cry of mourning was heard among them all”). For discussion of the grammatical motivation for the 1920 change to all of them, see under Alma 28:5.
Summary: Retain in Alma 11:13 the original use of them all in “an onti was as great as them all”.