Life of car tyre is ~40e3 km. 5.8g * 40e3km = 232 kg. Or 58 kg per tyre. A new tyre weighs about 8kg. I guess it might loose 3kg. So I think they've over estimated by about 20x.
Keep in mind too that a tire that has no usable tread life left is still quite heavy compared to the full tread life, new, version because there is so much left for the sidewall and the steel belts inside of the tire. They must be getting this number from measuring something else in addition to the tire