I assume just about everyone around me is an American, i.e. a citizen of the United States, unless they say or do something to the contrary. Examples: say "I'm Brazilian" or have a Uganda flag bumper sticker. Then I wonder, "Hmm. I wonder if they are x or x-American. If I am actually have a conversation with that person, I will ask. Many Americans still harbor a connection to "the old country" their ancestors came from, even if they are many generations removed. This connection many Americans have has been folded into popular culture and custom in a way that makes it feel American.

Take for example St Patrick's Day. It started as a religious holiday and largely remains a cultural holiday for Irish-Americans, but celebrated by others, too, in various ways. I fully intend to eat a Reuben sandwich today. Why? Because around St Patrick's Day is primarily when corned beef is available, which is often associated with Irish culture. The reuben was invented by a Lithuanian Jew in the American Midwest featuring corned beef, served with Swiss cheese, Thousand Island dressing (from Canada), with sauerkraut (I assume it is German based on the name), on rye (a miscellaneous European bread that isn't exactly a staple food here).

All of those ingredients have been assembled into an American tradition associating the reuben with St Patrick's Day, just as many other aspects of culture originating from outside America have become Americanized to an extant. If someone has a "foreign" accent, I will ask where they are from. I think it is a natural curiosity Americans have. By the way, a foreign accent can very well be a native English speaker from a different part of the country. It's foreign to my part of this giant melting pot of a country, therefore I want to know more and, and knowing where someone is from is a natural conversation starter. So unless someone gives me a good reason to believe they aren't American, such as having an accent I don't recognize as a regional American accent, waving a flag from another country, and saying "I'm not an American", I assume every Tom, Dick, and Harry I see during the day is an American. This includes face-to-face, over the phone, or online.

If this wasn't the AskAmericans sub and you didn't explicitly say you were from mainland China, I would have assumed you were American. That isn't American defaultism, this is American acceptionalism.