they're also measuring vastly more "emissions" over the life of the tire than there is material in the tires, so it appears that what they're actually measuring

they're also measuring vastly more "emissions" over the life of the tire than there is material in the tires, so it appears that what they're actually measuring is the asphalt dust that's being created by the interaction between the road surface and the tire - a longer-wearing tire would presumably be harder material and cause slightly more road wear as a trade-off to less tire wear, so if they had actually tested it a higher-quality tire might cause more of what they're classing as emissions. but you can't really make an assumption either way when you've only measured on a sample size of one.