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").

I'm not a fascist or a troll, and if someone wants to read exactly what DHH wrote, then they can click on the link to his blog post, which is in my summary. I think it's good that you're quoting more of it to be honest, so people can see what he wrote, and they can decide for themselves what they think about it. I'm not trying to "whitewash" anything, I just wanted to give people an overview of the situation so they can draw their own conclusions, whether positive or negative.