Greentext

I’m not American, I’m American

Assuming you meant "specifically not from the USA" instead of the second "American", with that being a typo (feel free to clarify)...

[N]o one living in the greater North or South Americas would say the above while speaking English.

... I loosely mentioned US exceptionalism, but I didn't know American Continent exceptionalism was even a thing - with the benefit of the doubt, I still don't.

"Statiunitense" is an Italian word that I first learned in a formal setting as an adult (18+) from a Fineco [finecobank.com] employee, which loosely implies that it's an appropriate way to refer to something from / belonging_to the USA in a small fraction of the geopolitical landscape...

insisting on the former feels like textbook UnitedStatesOfAmerican exceptionalism.

... hence my previous comment. It is possible to perform grievous acts of TheLandOfTheFree Exceptionalism without living in the Land Of The Free *, mind you.

"Usian" (USian, wgaf about capitalization) is as valid of a word as "Japan" is, refering to 日本 (pronounced "Nippon" - [source]):
some languages have words that semantically refer to YOUR interpretation of "American", WHILE having different single-word equivalents to what you would say when refering to either the North American continent or the South American Continent.

... with the latter two occasionally known as, y'know, "America".

Why bother differentiating between "American but not outside the jurisdiction of the United States of America" and "Either South American or North American, regardless of affiliation to the United States Of America", when you can just either say "American" OR "Usian"?

* 60 seconds later edit:
For many non-adult, probably autisticly coded years I looked up to the USA as a cool nation. I am no exception to Super Earth exceptionalism.