You are absolutely correct that distribution matters. However, Finland has an even more uneven population distribution than the US. 75% of the population lives in the costal cities, with 30% of the entire population living in the capital region( density of 193 persons/km²).
The entire rest of the country is not empty dessert ( which would require no services), but very sparsely populated rural woodlands, down to 2 people per km².
Density still is an overall useful quantifier given that extra knowledge, as providing services for a small population of only 5.6mio inhabitants is not easy either. Sure, providing coverage for the 75% in the cities is fairly easy. But that still leaves 1.5mio rural residents, which require huge investments in cable to supply with broadband. And due to the vast distances, you definitely cannot cover them with wireless alone, if you were thinking that.