I'm fairly certain all they meant to communicate was that Indians in India have different types of Indian culture to Indians in Britain, and that they weren't trying to reduce all of India to a singular culture.
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.
Really?
What is the 'one Indian culture'? I certainly can't pin down the a supposedly single Australian culture that we have.