The point of the article is not that there is an American language, but that there could have been one if Americans (and the British) chose to and allowed AE to

The point of the article is not that there is an American language, but that there could have been one if Americans (and the British) chose to and allowed AE to diverge significantly more from BE. People often consciously choose what words to use. In the end practical reasons - trade, business and mutual cultural influence kept the language from splitting up.