Skimming the article [0], it looks like they compared outcomes of people who lived closer to the road against people who lived farther away from the road, in the same building.
I think the idea is that people living in the same building should all have roughly the same particulate and pollution exposure, but people living on a road-facing side of the building will have more exposure to road noise. It's an extremely clever study design.
0: https://www.bmj.com/content/374/bmj.n1954