Often your culture isn't apparent and to you at all until you have extended exposure to another one.
And even then, it's hard to pin down.
I, for one, notice that Australians are quite different from Americans and Brits. And of course even more so from the multitude of Asian, South American and European cultures.
I think you have the wrong take-away from that sentence. The point was not to imply a singular origin country culture, but that the source culture and the immigrant culture end up diverging to some degree.
Whether the origin country has a singular culture of many subcultures is really irrelevant to that point. It's just a matter of whether the immigrant culture diverges from the source subculture.
And even then, it's hard to pin down.
I, for one, notice that Australians are quite different from Americans and Brits. And of course even more so from the multitude of Asian, South American and European cultures.