The committee for the 1920 LDS edition added the preposition of to this passage. This change reinterprets somewhat as a noun rather than as an adverb with the meaning ‘to some extent’. Near the end of the Book of Mormon, there is a second example of where the 1920 edition added the preposition of after somewhat:
The earliest reading for both the Words of Mormon 1:12 and Moroni 9:1 must not be especially egregious since the RLDS text has retained the original, more unusual phraseology. Moreover, there is one other example, in Moroni 9:24, where somewhat is directly followed by a noun phrase: “wherefore write somewhat a few things”. In this instance, insertion of an of would be less compelling; one might expect concerning rather than of to follow somewhat. The text has 11 examples of “somewhat concerning”. Even so, the adverbial use of somewhat in Moroni 9:24 seems acceptable enough and most probably read without concerning in the original text.
The text also has two examples where of is found after somewhat and before a noun phrase, but in these instances the of heads a prepositional phrase that complements the verb; in these two cases, somewhat acts as an adverb:
Thus are no examples in the earliest text of somewhat being immediately followed by of unless the of serves to complement the verb. The critical text will therefore restore the earliest readings without the of in the Words of Mormon 1:12 and in Moroni 9:1.
Summary: Remove the of that the 1920 LDS edition added in the Words of Mormon 1:12 since in the original text somewhat can serve as an adverbial with the meaning ‘to some extent’.