I'll just give you a quick overview of my thoughts, emphasizing as always that the people at Book of Mormon are great people. They just have a particular point of view that I don't share.
You won't be surprised to see the entire KnoWhy seeks to promote a Mesoamerican setting.
The main references are to Alma 49 and 50. Mormon describes the banks of earth that were thrown up around the cities, with well-defended places of entrance. There were ditches and "works of timbers" "upon the top of these ridges of earth." This was around 72 to 67 B.C., according to the chapter headings.
Despite the detail about the fortifications, Mormon inexplicably forgot to mention the massive stone pyramids the walls were supposedly defending, as depicted in all the artwork in the KnoWhy.
The KnoWhy offers as an example the "defensive earthworks at Becan (ca. AD 100)." Other sources put the date of the defensive construction at Becan at ca. AD 250. Readers of the Book of Mormon might wonder why such massive defensive earthworks were being built during a time in which everyone had all things in common and the people were living in peace. (See 4 Nephi.)
One answer: Mormon wasn't describing anything that happened in Central America.
The author of the study of Becan, David Webster, noted that "It requires no particularly inventive mind or prior tradition of military architecture to conceive of a ditch as an effective barrier, and the simple expedient of heaping up the excavated material to form an inner embankment is an immediately logical extension of the latter." Consequently, we would expect to find such fortifications "among many ancient cultures," as the KnoWhy recognizes.
So it should not be a surprise to also find them among the Native American Indians in North America.
For example, the complex found in Ohio on land owned by Mordecai Hopewell enclosed about 130 acres. (The Indians who lived there have been named after Mr. Hopewell. Had his last name been Nephi, everyone would be calling these ancient people Nephites.) The walls were originally 35 feet wide at the base. Archaeologists have determined that the embankment wall and ditch around the Hopewell site were likely built after mound building was well established at this site.
That lines up with what the text says. Captain Moroni had his armies "commence in digging up heaps of earth round about all the cities, throughout all the land which was possessed by the Nephites." Alma 50:1. Later in chapter 50, Teancum defeats Morianton. In Chapter 52, Moroni, Teancum and other chief captains have a council of war. They decide to entice the Lamanite armies "to meet them upon the plains between the two cities." During Zion's Camp, Joseph Smith identified Ohio, Indiana, and Illinois as "the plains of the Nephites." He passed within about 70 miles of the Hopewell site. Referring to the Nephites by name, Joseph said he and his men were "roving over the mounds of that once beloved people of the Lord, picking up their skulls & their bones, as a proof of its divine authenticity."
To Book of Mormon Central, none of what Joseph said is relevant.
While the Mayans in Becan were building defensive structures during the time of peace described by 4 Nephi, in Ohio, according to the National Park Service, "the ancient American Indians who built this sprawling structure [at Hopewell] were part of a cultural golden age that flourished in this region from A.D. 1 to 400." Give or take 30 years or so, it's a pretty good match to the Book of Mormon.
The Hopewell site was under cultivation for over 200 years, and archaeologists have not found any dateable features associated with wall construction. They can only observe the walls were built after the interior mounds were established.
Maybe that's why Joseph identified the plains of the Nephites for us.