Earlier quoted context omitted.
It doesn't seem clear at all that having many different kinds of Western Europeans makes for a "highly homogeneous" culture compared to neighboring countries that generally have only one kind of Western European.
The culture between Italy and France is much closer than Italy and Algeria. For as much difference there is between the countries in Western Europe, there are far more similarities amongst them compared to other continents and larger regions. Counting Italians as "immigrants" is at the very least slightly misleading.
The slightly racist assumption in America is that if I as a white Norwegian settle in America I contribute less to diversity than a black American who has lived his whole life in America.
In America diversity is all about what you see on the outside.