Haunted - Friston is on Independent SAGE, the self-appointed group set up to criticise the government that the Dementors love, so I think they feel especially betrayed by him. He already has their imprimatur and then he goes against their most fundamental belief.
Have you watched Friston's Unherd interview? It's actually quite boring, he's no Gupta or Levitt or Giesecke. (Jesus, what has my life become? 
)
He's SUPER cautious and incredibly careful to make sure it is understood that he is a mathematician, not a doctor and he is working with models and can only come up with mathematical interpretations of the data.
His main argument seems to be that according to his models, there is a high degree of resistance to Covid in the population. He uses the idea of immunological "dark matter" as an explanatory tools for why the numbers are working the way that they are.
It is up to immunologists and epidemiologists and virologists to figure out what that "dark matter" might be, but he thinks it varies across populations.
He says that Neil Ferguson's model actually stands up using his model, because if you run the Imperial model based on a high number of people resistant to Covid, you would get roughly the numbers we have now in reality. The mistake Ferguson made was assuming that there was no resistance in the population and that now (according to Friston) looks like a mistake.