When you drive, the tread of your tires becomes more and more shallow, and at some point, it's so shallow that they need to be replaced. It happens, because the tires get abraded as you drive, shedding some of their surface. Where do you think the surface goes? Tire pollution is very real. Nobody says that the pollution isn't caused by combustion: it's another major source of pollution.

Tire pollution is in some ways worse than combustion pollution these days. Modern gasoline-burning engines (and modern, properly maintained diesel ones) mostly emit volatile, gaseous pollution. However, tire pollution consists of solid microparticles, that contribute to significantly to airborne dust, which can be measured through PM2.5 and PM10.