Let's examine that a moment.
First, recall that Joseph explained "the title page of the Book of Mormon is a literal translation, taken from the very last leaf, on the left hand side of the collection or book of plates."
IOW, Joseph translated the actual plates. He didn't just read words that appeared on a stone in a hat.
[At BYU Education Week a couple of years ago, I was in a session on Church history when the instructor actually said, "We don't know how Joseph knew this because he didn't use the plates during the translation." That's how befuddled our LDS scholars are.]
Anyway, the Title Page explains the contents:
“Wherefore, it is an abridgment of the record of the people of Nephi, and also of the Lamanites … An abridgment taken from the Book of Ether also.”
Here's the link: https://knowhy.bookofmormoncentral.org/knowhy/why-is-the-book-of-mormon-called-an-abridgment
I do want to point out how BMC tries to fit Nephi's original record into the Title Page.
The prophet Nephi similarly referred to a portion of his writings as “an abridgment of the record of my father” (1 Nephi 1:17).
If you read the whole verse in context, Nephi tells us he's writing his own original account, starting with an abridgment of his father's record to explain how the events in his father's record affected him, Nephi, personally. This "abridgment" consists mainly of his father's dream. The rest of his account includes his journeys to Jerusalem (which could not have been part of his father's record unless his father wrote what Nephi told him), his building a ship, sailing to America, separating from his brothers, etc.
There is a stark difference between Nephi's original record (passed down all the way to Omni, with original writings all along the way), and the explicit abridgments by Mormon and Moroni from Mosiah through Ether, with Moroni sealing the account with his own few words.
You can see a graphic of the two sets of plates here: