I am sure that Margaret Atwood would have no problems about describing herself as an American in the same sense that people in Europe describe themselves as Europeans, people in Africa call themselves Africans, and people in Asia use the term Asians - which of course is a perfectly valid sense.

This involves no confusion, since there is no country in Europe, Africa or Asia were the name of a continent is used preferentially for its own inhabitant. America is the only continent where that has happened.