The Lord speaks directly to Joseph Smith through Nephi to provide instructions on how Joseph should handle the sealed text. Joseph is told to leave it untouched. After the witnesses have seen the book, it is to be hidden up again. In this case, the “seal” is once again the symbolic as much as the physical. Not only is Joseph to leave the physically sealed portion of the book, but the entire book will become “sealed” again when it is returned to the Lord.
Textual: These three verses, along with verse 20, present an interesting change in the flow of the narrative. They are absolutely an interruption in the story, a shift from Nephi speaking to a generic audience to the Lord speaking to Joseph Smith. Regardless of how we understand the plate text, these verses are written directly to Joseph Smith, and are direct instructions to him.
We might speculate that the translation method allowed the Lord to interrupt the plate text for this very specific command to Joseph Smith, based upon the text that was being translated. In a very real way, this would treat Nephi’s text just as Nephi treated Isaiah’s - as a springboard to revelatory communication. Of course it is equally likely that the text was part of the prophecy. There is no way to tell, but neither interpretation of these verses violates the general integrity of an inspired translation of an ancient text.