Oliver Cowdery accidentally miscopied this as the. The demonstrative pronoun clearly improves the reading of the text. Moreover, it agrees with the previous use in the verse of this land in “behold it is wisdom that this land should be kept as yet from the knowledge of other nations”. Another example of the tendency to replace “this land” with “the land” is found later on in the text:
Here the 1830 typesetter is the one responsible for replacing this with the.
Summary: Restore in 2 Nephi 1:8 the reading of 𝓞, with its original demonstrative this (“many nations would overrun this land”).