It is referring to the total from all four tires i.e. 4 kg per car per year [1].
Please note that car tires or anything really won't have uniform degradation. The first year will lose a lot of mass, then once the outer material is shed, there will be far less shedding [2]. So I understand the above number as global tire shedding in a year/total number of cars.
P.S. I just looked it up. I have no idea if the source is reliable.