From the article you cite...
“The fact that a rural, broadly Midwestern dialect became the basis of what is General American English is often attributed to the mass migration of Midwestern farmers to California and the Pacific Northwest from where it spread,[citation needed] since California speech itself became prevalent in nationally syndicated films and media via the Hollywood film industry.”
Having lived in CA all my life, it always seemed to me that people on TV talk like people here do. Most people from the Midwest sound “hick” (as Midwesterner Rush Limbaugh calls it) in comparison.
Yes, for a variety of reasons the midwestern accent took over CA, especially SoCal where so much of the entertainment industry is headquartered and so many of the people think they’re going to be famous. It’s the midwestern accent that rules the media, since long before CA became the media capital.
No matter how you want to trace it, the General American Accent, which is what the media uses, came from the midwest.