My take is that a lot of USians will vociferously defend the differences in regional cultures within the US - East vs. West, coastal vs. flyover, state vs. state and so on - as both a conscious and an unconscious response to questions of national cultures and norms. When someone claims the US is "too big" or "too diverse" (and isn't using the latter as a dog whistle) you see some of this implicit balkanisation of the US into incompatible units.
Of course the consistent reaction of Americans to the use of terms like "USians" is a big giveaway to the fact that there's a national identity and thus culture. As, I guess, is the habit of down-playing of any such thing lol.
Maybe I've just traveled to much, and I have certainly rolled my eyes at some of my less informed countrymen, but I have a hard time believing people could think that our culture is somehow a baseline.
I agree that we project our culture a ton, and I really wish we wouldn't. It's so annoying to try and share something cultural with a foreign friend only to find out they already know and have opinions about it because they saw it on Friends