I think you're being unfair. To say Arlington and Flint differ is not to say Flint and Peoria do not differ. If anything, Flint is being used as a stand-in for similar struggling communities in the non-costal, non-urban US.

Also your "more compelling argument" is contained within Summer's argument: Flint is way worse off than Arlington, this is bad, here are some ideas to fix it. He even goes on in the last paragraph to say:

These may or may not be the best ideas for enabling people wherever they live to share in U.S. economic progress, and they are no substitute for addressing inequality more directly. But it is hard to see how we can bring about enduring improvement in the nation’s condition without addressing the needs of the tens of millions of Americans who live in places that are failing to catch up with the rest of our country.