1. London when the ULEZ was brought in was already a lot less polluted than historically.Are there gains from that? London was not that bad by global standards.
2. This was followed by the benefits of COVID lockdowns (not a phrase often used!) as pollution dropped much further.
3. Is this effect even across London? Suburbs vs centre, different types of housing etc.? I grew up a the edge of London and the difference in pollution was visible (i.e. you did not need instruments to see signs of it).
4. There was also a reduction in sales of diesel cars which had been heavily encouraged (because of lower CO₂ emissions) in previous decades.
5. As the articles notes, there have been a lot of other changes at roughly the same time. It only mentions one, but there have been significant lifestyle changes, so a lot of potential confounding variables.