My understanding was always that tires produce a lot of particulates but they are fairly large and heavy; most of them fall out of the air very quickly.

If that's true it could be true that cars emit lots of PM from tyre wear but must of it never reaches detectors and can reasonably be ignored. That doesn't necessarily invalidate the overall point: that we reduced PM from exhaust enough that other still unregulated sources are better targets for future reduction efforts.