Jehovah grants Nephi the power to command the elements and to have them obey. That is a fundamental definition of the power of God. Thus, he can “say unto this temple it shall be rent twain, it shall be done. And if ye shall say unto this mountain, Be thou cast down and become smooth, it shall be done.” These are powers far beyond human capacity, but within the power of the God of the earth.
Verse 7 also speaks of sealing and loosening. Many might have authority to seal or loosen on earth. That depends only upon an authority granted by man. Nephi’s authority can extend beyond mortality, and he can seal or loose in heaven as well. Nephi has the ability to tap into the powers of heaven.
Modern saints understand this sealing power in the context of ordinances that may be performed on earth, but continue to have validity in the heavens. That is true doctrine, but might not have been fully revealed to Nephi. The context of what Nephi will seal is more part of nature. He will give the command to seal on earth, and the heavens will accomplish that command. Note that Jehovah tells Nephi: “If ye shall say that God shall smite this people, it shall come to pass.” Nephi can initiate and promise, but it is Jehovah who acts. Nephi can say that a mountain should be cast down, but Jehovah will do it.