There is not difference between the meanings of 'an American' in either of the sentences, in my opinion.
It was not the American who made the rude comment. The American was saying that he would have responded with an encouraging, kinder statement.
It was a native like a Japanese or a Chinese,or whaetever in a country where the American student stayed. And the native speaker in the foreign country scoffed at his poor foreign language like Japanese. So the writer felt if an American had responded instead of the native Japanese, he would have said more politely.
So my question was if "An American would not use that word" can have the interpretation of "If he/she were an American, he/she would not use that word", and if it's proper.
ex)...An American student tells the story of how he was surprised when he was in a foreign country. He
said to a native, "I don't speak your language very well." The native replied, "I should say you don't." An American would not use that word. An American would have commented, "Well, you have only been here two months." or "But you are making progress."...
Once again, keannu, you are trying to pin down an exact equivalence between two different expressions. This may or may not be possible - I don't know, and I don't really care. If my italicised words come across as dismissive, I think that you need to know that most native speakers do not analyse their utterances very deeply. We might say one thing on one occasion and another on another.
Thanks a lot! Anyway, I had the same feeling as you, and I could understand it comparing it with the corresponding Korean expressions. It was an example of my grammar book and I had to explain about it responding to my students' questions. They asked me "what does "one" here mean? What does this sentence mean?" and I had no way to explain very well, feeling stupid.
Sometimes I don't really like such parts of English grammar books written in Korean, no rather I'm confused as I don't know if the examples are correct as they might have been created by Korean authors or native ones.
Sometimes I feel like teaching grammar materials written in only pure English, but students will have a hard time understanding them.