As I understand it, the Kansas City facility is for the pre-inspection of goods traveling from the United States into Mexico. It mirrors facilities in Mexico that the United States uses for the pre-inspection of goods entering the United States from Mexico. It is NOT Mexican Sovereign territory. US laws will apply there.
If it is not going to be the sovereign territory of Mexico (where the Mexican customs officials will be doing their inspections) then there is no need for the State Department to be involved.
Additionally, Mexican customs officials can only enforce their law on their territory. Our pre-inspection facilities at airports, etc. are the sovereign territory of the U.S. If it wasn’t we could not enforce our Customs laws.