I’ve seen posts online or memes about it, mainly discussing American people who think the internet as a whole is US-centric despite the topic at hand taking place in another country (nothing to do with America), for example: someone wrote the date format as 30/10/2020 (using the EU format) but some American was like “THE CORRECT WAY IS 10/30/2020” when they fail to realize that user is not American at all.
Is US defaultism real when they travel abroad? They assume the $ is legal tender worldwide in countries where their own currency is used more (the only thing that’ll work is exchanging it but not for direct transactions). I’ve heard an American woman went on a tirade whilst in France as the cashier refused a $20 bill when the prices are in EUROS (WTF?) or being like “SPEAK ENGLISH!” in non-English speaking countries.
Regarding your comment regarding the Internet, US defaultism generally happens as Americans are a plurality of English speaking people on several major platforms and a lot of news in non-English speaking countries is usually written in the local language and discussed in the local language. So, the assumption is generally that the person is American, especially if they are taking a point of view somewhat in line with American politics.
I’ve seen a lot of angry comments from too online leftists on Lemmy telling the person they are talking with that they hope the other person dies in a fire as America falls, with the response being “I’m not American”.
A lot of people understand American politics because it is written in the language in which the Internet was developed and so much is written. Without other context clues, people may not see that someone within the boundaries of American politics may be from another country.