Moroni has used the argument of the unchanging nature of God as part of his conclusion to his father’s text:
9 For do we not read that God is the same yesterday, today, and forever, and in him there is no variableness neither shadow of changing?
This is apparently an argument that flows from Moroni’s understanding, and he repeats the general argument here. We are unaware of the time separation of Mormon 9 and Moroni 10, but we it is likely that it is multiple years, perhaps even twenty. This suggests that this is an argument that is fundamental to Moroni’s understanding, and not simply a passing preference that is used for a time.