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.


Firstly because when he says London is "no longer full of native Brits" he links to a Wikipedia page about ethnic groups in London. So he seems to be talking about ethnicity. Secondly, he says that these "native Brits" are now "about a third" of London. Looking at the Wikipedia page he linked to, the stats show that white Brits were 37% of London according to the 2021 census, which is about a third. It seems to me that's probably what he's talking about.
If "native Brits" just meant people born in the UK, or people with a British passport, then those figures are higher: 59% and 77% of London respectively (that page says that "In 2021, more than 4 in 10 (40.6%) usual residents in London were non-UK born, and more than 1 in 5 (23.3%) had a non-UK passport").