The pronoun “they” in verse 5 refers to the prophets of old who Jacob had mentioned in the previous verse. Jacob is grounding the Nephite believe in the atoning Messiah to the old, and venerated, prophets. Those prophets had prophesied of the coming Messiah, and they had also kept the law of Moses. Thus, the Nephites, who also prophesy of the coming Messiah, also keep the law of Moses.
The plates of brass contain Jehovah’s word to past prophets, and Jacob declares that while they were distant in both time and space, they continue to be relevant for the Nephites.
The idea that one might command in the name of Jesus and have the trees, or mountains, or the sea obey, is related to concepts they learned from the plates of brass. Although the verse uses the name Jesus, that is probably a translator’s choice. For Jacob, the Messiah was Jehovah, and Jesus was the name for Jehovah come to earth. Understanding that Jacob intended Jehovah here is important because it gives us the context to understand why he says that nature obeys Jehovah. Jehovah is the god of nature in the Old Testament. Isaiah had spoken of Jehovah’s defeat of Rahab, who was a primordial monster of the sea. Later, in the New Testament, Jesus will also tie himself to this Jehovah who is dominant over nature when he calms the storm in the Sea of Galilee.