You can't go by pure density. Imagine if you transported Mexico City into Antartica. It would be sparsely populated, but highly concentrated. Cellar coverage would be easy.

Are rural areas in Sweden dominated by small, fairly dense towns? In America, even small cities are sprawling because of cars. People will live 10 miles outside of a small town of 3000 people.

Rural areas in Sweden and Finland are dominated by forests, swamps and lakes. You've got 3G or 4G practically everywhere.

And mountains, not to forget. Though the coverage up in the mountains is not always great if you are away from populated areas.

Yes, real mountains in Sweden and fells in Finland. Still, where you have a dwelling, you usually also have coverage.