"The bulk" is overstated, but they have certainly done a lot to shape global perceptions of italianità (particularly in cuisine and pop culture) and the complex transatlantic back-and-forth between emigranti in North America and Italy was crucial for 20th century Italian culture.

'The periphery is the real culture of the country, while the core is overlooked (if not fake and anational)' is a common trope, very familiar to this Irish person.

The greatest (in a value-neutral sense) contribution of Italy to twentieth-century culture, the invention of fascism, happened in the north.

This whole "most Italian culture is actually Italian-American" meme is a massive overcorrection to a genuinely problematic and simplistic nationalism and mythologizing in Italy, particularly in cuisine.

The northern migration within Italy after WWII by southerners was also crucial for shaping the current "national" cuisine as well.