I guess this is not that surprising. Globally, rich countries produce more emissions than poorer countries (per capita).
I did wonder whether EVs would have come into play here, but their 2015-2019 time period may be a little early for the increased EV adoption of the last 5 years or so.
Another factor is that there are a lot more people working from home now, and these jobs tend to be office jobs like IT that probably pay more. That should reduce a lot of transport emissions.
It would be interesting to see this study repeated.