In the linked article, DHH links out to a wikipedia article titled "Ethnic groups in London"[0].
He then uses a statistic that "only a third" are native brits in 2021, which roughly lines up with the "White British" line in the chart.
You can argue that "white supremecist" is a charged and problematic term, but I'd say that "Here he complains about too many brown people in London." is a fairly accurate representation of the article. I'd say "disgraceful slander" is a bit too strong as a rebuttal.