I wasn't trying to invent a dividing line, more trying to highlight how usage/understanding can vary. Here's some discussion from government statisticians:

https://www.ers.usda.gov/topics/rural-economy-population/rur...

Large swaths of the US are what that page calls 'micropolitan', centered around small urban clusters, which are the areas I think cause the most confusion, built up areas that are treated as urban in statistics and conversely thought about as rural by lay audiences.