I have some very heavy doubts about the validity of their data. Look at their primary findings:

"Using a popular family hatchback running on brand new, correctly inflated tyres, we found that the car emitted 5.8 grams per kilometer of particles."

Most tires seem to be around 7kg, which means according to their data a car would have all 4 tires reduced to nothing after 4828km. With most tires going bald after ~50,000km (not fully vaporized mind you), there's clearly something wrong here.

My guess is they tested brand new tires which may shed particulate quickly at the start, similar to how a new carpet sheds exponentially more at the start.