Verse 30 is the continuation of the concept that Moroni began at the end of verse 29. After the comfort, Moroni understands that faith is the mechanism. However, he also knows that faith is effective. Unlike some modern men who claim that faith has no power, Moroni clearly understands faith as a monumental transformational tool. It is a transformational tool so great that even the face of the earth might be transformed through the relocation of mountains.
Redaction: Moroni tosses off this incident as through it were something with which we were familiar. It is the only time that this instance is mentioned. Since it deals with the brother of Jared, we may safely assume that it was part of the plates of Ether, as that is the only record we know about for the brother of Jared.
The fact that Moroni mentions it here also tells us that he has heavily edited the story of the brother of Jared. Here is an entire instance that must have been related in the original source, but of which we have only this mention in passing. It is simply a witness to the amount of material that Moroni may have left out of the plates.