Verses 11 and 14 bracket the middle verses with Jehovah’s command that Nephi declare that the people will be smitten unless they repent, and the actual declaration to the people by Nephi.
In between, Mormon tells us that Nephi went out preaching, and “declare[d] unto them the word of the Lord, concerning their destruction if they did not repent.” In other words, he did just what the Lord told him to do. Verse 13 tells us that even though there had been a miracle to which many could attest, “they did harden their hearts.”
When verse 14 repeats the information that Nephi declared the word of God to them, it was as a final declaration that the penalty was to be imposed. They had had an opportunity to repent, and did not.