People that die of lung cancer don't have 'cigarettes' on their death certificate either. Just like people that die of NOx damage to lungs have respiratory failure and not NOx as the cause.
That PM2.5 and NOx levels in Central London have approximately halved is no argument because the ULEZ expansion is about inner London where they've only reduced by ~ 20%. And of course that halving in Central London was in no small part due to the ULEZ.
As I said there are no easy answers here. People bought these pre 2015 vehicles with a reasonable expectation of being able to use them for 10 years but ignoring the deaths they net cause is no way forward either.