I think it’s a dodge to answer the question about why U.S. broadband is so much worse than Japanese or South Korean broadband by referring to America’s lower population density. If the issue were that we have excellent internet service in some places, but it’s super-slow in Wyoming then, sure, you’d say it’s the density. But the population density of Southern California or, say, New Jersey is pretty high and you don’t get Asian-style broadband there, either. For that matter, you don’t get Asian-style broadband in Manhattan.
The relevant issues here are regulatory in nature — Japan has smart regulations that produce quality service, whereas the U.S. has regulations that are good for incumbent telecom firms. It’s true that there would still be a real residual issue related to people who live in very low-density areas, but that’s a separate issue from why the majority of Americans who live in metro areas can’t get decent broadband.