From what I've read in articles debunking the whole NAU scenario, the KC SmartPort will NOT be Mexican territory, but there will be a Mexican customs office to clear traffic heading south into Mexico.
My opinion (complete speculation) is that the offending passage was written by someone who didn't understand the difference between an office on sovereign U.S. territory operated by Mexican officials, and an office on U.S. territory operated by Mexican officials and thus sovereign Mexican territory.