The brother of Jared was “mighty in writing” (Ether 12:24) while Nephi was mighty in speaking. Nephi was not aware of the writings of the brother of Jared at this time so he was not making a comparison as Moroni did in the verse cited above.
The Holy Ghost, as a teacher, carries the message of the speaker unto the listener’s heart, or testifies to him. The Spirit may also testify to the speaker that the listener knows the message he is hearing is true “by the manifestation of the Spirit”(Alma 7:17).