Oliver Cowdery initially wrote “the practice of the Lamanites”, but then virtually immediately he corrected “the Lamanites” to “these Lamanites”. The 1830 typesetter, however, reverted to the the that Oliver had originally written in 𝓟. Perhaps the many occurrences of the in this sentence (“the practice … by the waters of Sebus … the flocks of the people”) led to the persistence of this error. Moreover, one could conceive of the practice of raiding others’ flocks as a Lamanite practice in general. Since the correction is virtually immediate (there is no change in the level of ink flow for the se that was squeezed inline between the the and Lamanites), the critical text will restore the original these. The 1981 LDS edition restored the corrected reading these, but the 1908 RLDS edition did not, even though the reading is found in 𝓟. There seems to have been little motivation for Oliver to have edited “the Lamanites” to “these Lamanites”; the use of the seems quite innocuous here, especially when we consider the fact that no printed edition made the change to these until the 1981 edition (and there the change resulted from consulting 𝓟).
Summary: Accept Oliver Cowdery’s corrected reading in 𝓟, “it was the practice of these Lamanites”; the correction appears to have been virtually immediate, and it makes perfectly good sense.