The front-page article on this site is "8 Surprising Facts about James Dean," so I wouldn't take it too seriously. It reads like a high-school or undergraduate essay. Also, FWIW, it's from 2012.
Hopefully a linguist will come along with better treatments of the subject, but Google turned up a couple of articles with more links:
https://curiosity.com/topics/the-american-accent-is-older-th...
http://www.bbc.com/culture/story/20180207-how-americans-pres...
And, of course, there's Wikipedia: