I think the source of our disagreement is in our answers to this question: is the USA many subcultures under a common umbrella (“American culture”), or is it many independent cultures mixed together?

I could be convinced of your position if you can name the common elements that justify the umbrella. It becomes even stronger if those common elements are the rotten ones.

Anecdotally: I married someone from a very different (USA) culture than my own, and their family feels as different from my own as some people that I’ve met from other countries, so that’s partly why I answer that question the way that I do.