Why doesn’t the Book of Mormon include “Ariel” when quoting Isaiah 29:7?

In 2 Nephi 27:3 we read:

And all the nations that fight against Zion, and that distress her, shall be as a dream of a night vision; yea, it shall be unto them, even as unto a hungry man which dreameth, and behold he eateth but he awaketh and his soul is empty; or like unto a thirsty man which dreameth, and behold he drinketh but he awaketh and behold he is faint, and his soul hath appetite; yea, even so shall the multitude of all the nations be that fight against Mount Zion. (emphasis added)

This is curious because it is different to the King James Version of the scripture (Isaiah 29:7) which reads:

And the multitude of all the nations that fight against Ariel, even all that fight against her and her munition, and that distress her, shall be as a dream of a night vision. (emphasis added)

Robert F. Smith explains why this is interesting:

King James “Ariel,” a poetic term for Jerusalem, is not to be found in the 2 Nephi 27:3 quotation of Isaiah 29:7. However, it is also absent from the Jewish Aramaic Targum—which replaces it with “the City.” The Book of Mormon reads Zion instead. This fits well, however, since “Mount Zion” appears at the end of the verse (Isaiah 29:8), and “Zion” and “Mount Zion” parallel each other here.
– Robert F. Smith – Textual Criticism of the Book of Mormon

Where would Joseph Smith have gotten this alternative (yet appropriate) translation from?

See:

Robert F. Smith – Textual Criticism of the Book of Mormon

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