If they're actually uninhabited, then there is no need to build fiber out there, and you shouldn't count them in the density measure. What you're really interested in is the population-weighted density (i.e. the density of the places where people live).
Compare the list of U.S. urban areas to that of Sweden: https://en.wikipedia.org/wiki/List_of_urban_areas_in_Sweden; https://en.wikipedia.org/wiki/List_of_United_States_urban_ar.... The vast majority of the population of Sweden lives in urban areas with between 2,000 to 3,500 people per square kilometer. No urban area in the U.S. is as dense as Stockholm's (3,500). NYC is at the low end of that range (2,000), and is less dense than a bunch of Swedish cities with just ~50-100k people.
Major U.S. urban areas like Atlanta, Boston, Charlotte, and St. Louis are under 900 people per square kilometer. Major cities like Houston, Dallas, and Philly are just north of 1,000. That's 1/3 the density of Stockholm.