Informative article, but slightly inaccurate. General American is not "considered generally confined to a small section of the Midwest." This is a myth that I've heard a lot of Midwesterners propagate. Prior to the Northern Cities Vowel Shift, the most identifiable broadcaster accent was probably the Inland North dialect, which is spoken mostly in western New York, northern Ohio, Michigan, northern Illinois, and Wisconsin. That's presumably the source of the myth. But today, that accent sounds very different from most other dialects of American English (for example, their short 'o' as in "block" sounds almost like most American's short 'a' in "black" and their short 'a' sounds like the non-rhotic diphthong that speakers of RP use for "air"). I've heard Iowans claim that their accent is the most standard, but that couldn't be further from the truth, as most Iowans have backed 'o's and 'u's a la Minnesota, in addition to subscribing to the cot/caught merger, etc. The reality is that there are seven or so dialect regions in the U.S. with lots of variation from place to place within those regions. Some metropolitan areas have their own distinct accents. In my experience, broadcasters tend to use features from their native accent as long as they aren't from the South or a region heavily affected by the aforementioned Northern Cities Vowel Shift.

One other inaccuracy is the article's claim that the British elite had much less influence in manufacturing hubs in the Mid Atlantic where Scots-Irish immigrants and nothern English speakers of rhotic dialects predominated. One specific example they give is New York. In reality, New York English was non-rhotic up until recently, and even now, a sizeable number of speakers either have a non-rhotic accent or drop their 'r's in certain environments. I don't know the history of the New York dialect or whether it become non-rhotic at the same time the Boston dialect did, but I'm assuming New York English was influenced more by Dutch, Italian, and Ashkenazi Jewish immigrants than by Scots-Irish and northern English immigrants, which would explain why it didn't follow the rhotic trend of the other Mid Atlantic cities that the article talks about.