I think DHH is talking about ethnicity specifically though, not culture. He said London is “no longer full of native Brits”, and he describes “native Brits” as now forming “about a third” of London. White Brits were 37% of London at the 2021 census, so I think that’s what he’s talking about.

It’s possible to express disagreement with a cultural practice without making it about ethnicity. E.g. someone might object to men and women being separated for religious prayer. They might believe that men and women should pray together. It’s possible to make that point without making it about ethnicity.