The difference of domestic emissions caused by city vs non-city residence is quite minor, about 10% per person looking at UK numbers. In comparison, a single flight by plane can double a person emission.
It also not clear if using existing numbers of rural living makes sense since most people do not work from home. If the norm is to travel to work, the further people live away from said employment the bigger the footprint will be. Existing numbers reflect that culture.
Looking at CO2 per capita, does the very high density cities of japan have better numbers than say India? No. Japan has 10 times higher than India. Of course we all likely know that that wealth influence emissions, which mean it not that simple to just say that concentrated metro areas are better for the environment than sprawl. Emission rates depend on multiple factors.
I think we are both saying the same thing. And 10% difference is not negligible.
My point is that we should do everything we can to keep densification naturally happening, as it is better. This densification is not a permit to allow ourselves to pollute, consume more.
I'm not saying people should stop living there, but stopping densification will not help curb climate change and sustainable environment.
Also, not everyone wants to live in non-dense places.