You could change the instances of "American English" in the proponent's argument to "Australian English", make a few cultural changes, and the argument would be the same. He is not even presenting the argument for American English. He is arguing that there should be a standard variety, and apparently he feels it not worth serious argument that it should be AmE rather than any other dialect.
About the topic:
In any case, American English is already too diverse to be a standard.