Earlier quoted context omitted.
The notion that Sequoyah was a monolingual Cherokee is dubious. He had a European father (though he was raised with his mother) and worked as a trader and served in the U.S. Army. His cousin, to whom he presented his syllabary, was also half European, “George Lowery.” He had extensive contact with Europeans. Moreover, his syllabary includes adaptations of Latin, Greek, and Cyrillic letters. Part of the story is that…
I found it interesting that you used the term European several times, but never once the term American. He served in the American military, lived in America, had an American father (according to the article). So you consider 19th century America to be Europe, or is there another reason for your choice of words?
Foreigners can serve in the US Army. Native Americans weren't automatically US citizens until 1924, but were considered citizens of their sovereign tribe.
European here clearly means both "from Europe" (eg, Latin, Greek, and Cyrillic letters are European, not American), as well as "European Americans" (ie, Americans of European ancestry, and often with cultural ties to Europe.) Just like how "Asian" doesn't always mean "born in Asia", or how "Anglo" can refer to non-Hispanic white Americans rather than being specifically related to England.
Trading with the Spanish in Florida, English ships, or French trappers would all count as "contact with Europeans", and not simply "Americans".
Finally, recall that at the time "American" was a state of mind. A Loyalist at the time would not consider themselves "American", and a Patriot considered a Loyalist to be "inimical to the liberties of America". How do you know if Sequoyah’s father was an American or a Loyalist?