I agree. But tires do shed a lot. Consider a brand new tire has grove depth of about 8-10mm, and many drivers change tires when their groves are 0-2mm (yes, some idiots keep driving until they have slicks or "a pop").
Turns out you don’t need to measure air pollutants, you can just divide lost rubber by the distance travelled. It won’t be anywhere close to what was measured, but then again actual particles are a much more useful number because you can’t breathe a worn down groove.