AITAH for referring to someone from the United States as “American”?

I was having a conversation with a friend of a friend (who is from South America) about one of my co-workers, and I said something like “He’s born in England, but his mannerisms are very American.” She gave me this puzzled look and said “What kind of American? Like, from Argentina or Brazil?” I then said “No, American like the United States.” she looked upset and said “You know, America is a continent, not a country. You should really say from the United States, because America is 35 countries. Did they not teach this at your school?” It was a tense mood from that point on. AITAH? To my knowledge, the only non-colloquial demonym for the United States is “American.”