Here we have a typo in the 1849 LDS edition where the typesetter, it would appear, misread there were as they were (the text had earlier been edited for the 1837 edition from there was to there were). The source for this typo was probably the preceding they were (“and thus they were prepared to meet the Amlicites”). The correct reading with there was restored in the second printing of the 1852 LDS edition, probably by reference to the 1840 edition.
Summary: Maintain the use of there was in Alma 2:13, the reading of the earliest textual sources.