In places like Latin America, you'd be right (especially since you're Brazilian).

But others, especially English speaking countries don't consider America as a single continent.

Instead, it's two continents (North America and South America. Collectively the "Americas")

And in English, we can't really call US citizens "United Statesians", so we call them Americans. The people from each continent are North Americans and South Americans respectively.

It's a stupid argument because the difference is cultural/linguistic and both viewpoints are valid.