Both verses 11 and 12 emphasize the records. There are a couple of possible reasons for Mormon’s emphasis on the records. First, he is working with those records, and they form the foundation of his writing (our current text of the Book of Mormon). Mormon would have had to have read the large collection of records, and referred to them often during his writing (particularly for the sections where he appears to copy from them directly, such as the quoted speeches). Mormon’s sensibilities to the records of the Nephites would therefore be quite high. It is probably in this context that he adds that the records are true. This is a testimony he adds which highlights the truth of the record he is writing.
In addition to Mormon’s relationship to the records, the very fact of the records appears to be something that sets the Nephites apart from the Lamanites, and in more than just the writing. The records become the proof of right-of-rule, a marker of Nephite rulers. They also become the keepers of the heritage, keepers of language, and most importantly the keepers of their religion, which also means politics and science (see Mosiah 1:3-3-4).