I think that the article is saying they have a disproportionate impact on traffic. They are more directionally-biased, spend more time on the road, and have a g

I think that the article is saying they have a disproportionate impact on traffic. They are more directionally-biased, spend more time on the road, and have a greater spatial and temporal overlap with other drivers' commutes. All of those things apparently contribute to traffic more than people who travel in random directions at random times. Also, it means that it is (potentially) more of a fixable problem. If you can target those specific sources and destinations of problem drivers, you can try to offer them alternatives. Even if they are replaced by other drivers, many of those other drivers may behave more randomly, and the roads will have better utilization.