This simple verse reminds us who is abridging this work. The great Nephite commander Mormon never gives us much detail about the military successes of the Lamanites. While he will expound for quite some time on the military genius of Moroni and his captains, he just can't stomach a detailed description of the 'cunning of Amalickiah' (v. 27). Accordingly, we read of an entire Lamanite campaign in only a few verses. Of this entire campaign, the only detail we get is the part about the bold stance of Teancum's army and the bolder mission to assassinate Amalickiah.
And so, this seemingly insignificant verse is really a testimony that the Book of Mormon is true. It is just one of a thousand other unmentioned internal consistencies which could not be expected if the Book of Mormon was merely the elaborate concoction of an uneducated 24 year-old farmer.
(It should be noted that while the city of Nephihah was included on this list of cities, it was not captured by the Lamanites for 5 more years. See Alma 59:5-9. It is presumed that all the cities but Nephihah were captured in the year 67 BC, but that Nephihah was not attacked until 62 BC.)