While that may be true it’s very much in the same way that Roman culture is the basis and founding of Modern Romantic cultures. It’s to degrees and the more derived and hybridized the individual cultures are the more distant they become, not even accounting for cultural drift by time alone.

But I will also note that I don’t think the US has a united culture and what is often seen as a Pan American culture is moreso a side effect of the post World Wars migration, the migrations of the great depression/dust bowl, and the yuppies migrating about creating a rough amalgam culture that often out populates the older more divergent regional cultures. Basically if you think of a cultureless suburbanite that’s the thing I’m talking about. Also I think mass media reinforced the whole thing.