You are not comprehending the facts behind what the poster is saying. There are large parts of the midwest that were settled almost exclusively by communities of central and eastern European settlers in the late 19th century. These communities are culturally homogeneous because it was virgin agricultural country before these groups of immigrants settled there. No one else has ever tried to move into these areas, largely because it isn't that attractive to anyone but members of these communities. Most people that live there today are direct descendants of the original immigrants. There was no redlining, racism, etc -- that would kind of require that someone other than these people ever lived in these communities.

I've lived in the midwest in a small farming town where virtually everyone, mostly only excluding my family, was directly descended from people that moved there from the same small part of what is now the Czech Republic at around the same time. It was incredibly homogeneous culturally.