South American battleships were not bound to a 14-inch limit

How would a 14-inch gun restriction have affected battleships built for South American navies?

He says no such restriction controlled those orders; a South American customer could have obtained a 16-inch battleship if it wanted one.

The question assumes a universal 14-inch gun-calibre restriction that did not apply to South American orders. Builders would offer the required design, including a 16-inch battleship if a customer chose and funded it. Export customers were therefore governed by their purchasing choices and regional ambitions rather than by the treaty shorthand in the question. This makes the buyer's requirements and budget the decisive constraints.