According to wikipedia, they have been around since early pleistocene, about 2 million years ago. So their contribution has been part of a sustainable equilibrium for that long.
The study is actually titled "Unrecognized threat to global soil carbon by a widespread invasive species". So perhaps they aren't part of the equilibrium on some continents. The title of the news item is silly though as if suggesting that we should kill off the whole species and build another million of cars.