Yes, the census "80% of people live in cities" thing really just means "incorporated areas". In reality ~2/3 of housing stock in the US is detached[1]. Traditional detached houses and mobile homes combined are around 78 million. Now I have not seen detailed statistics about the exact number of people living in each type but it seems pretty safe to say that the average household size for detached housing is at least as high as for apartments, and most likely higher. US average household size is 2.58 people which would imply ~200 million people in detached housing, well over half the population.
[1]https://www.census.gov/hhes/www/housing/census/historic/unit...