Literary: Nephi adds his own poetic reversal to that of Isaiah. The hidden is revealed. Darkness is brought to light, the sealed is loosed. In each of these cases Nephi provides a symbolic opposite that represents this alteration of the normal world in the last days.
Textual: This conception of the revelation of hidden things appears later in Mosiah:
17 But a seer can know of things which are past, and also of things which are to come, and by them shall all things be revealed, or, rather, shall secret things be made manifest, and hidden things shall come to light, and things which are not known shall be made known by them, and also things shall be made known by them which otherwise could not be known.
These particular words are part of Ammon’s discourse before Limhi. While it is entirely possible that this is an independent usage of the phraseology (or that it was one influenced by Joseph Smith’s vocabulary and thus the similarity comes through Joseph Smith) we must examine the possibility of Ammon’s access to the small plates.
The small plates followed a family transmission trajectory through the end of Omni, at which point the plates are full, and they are given to Benjamin. It would be quite logical to assume that Benjamin and Mosiah at the very least would have read this record, presuming that it would be new to them. Thus Ammon as a son of Mosiah may also have either had access, or had the record read to him.
The similarity of terms in Nephi and Ammon thus have multiple possible means of connection, from random to remembered.