If one reflects carefully on the left today, what Salman Rushdie says here is obviously true.
Extreme multiculturalism is not just multi-racialism. Multi-racialism as a principle is right. The view that a country should not discriminate against people on the basis of skin colour or race is entirely right and moral.
But culture is not race. So many cultural ideas are not biologically determined, and are flexible and changeable, even if no rational personal would deny that certain human traits and propensities are a complex mixture of biology and environment (e.g., height).
More cultural diversity is not necessarily a good thing for a society, if this means more extreme religious fundamentalism, parallel legal systems and the introduction of values that radically conflict with the core values of a secular Western society (e.g., theocracy, gender segregation, homophobia, female genital mutilation, misogyny, etc.).
As Rushdie says, extreme multiculturalism is cultural relativism. This is a very bad thing.