Democratically elected leaders can represent their country, but they can also do things on their own. When Emmanuel Macron eats at a restaurant we don't typically understand the nation of France to be involved. Trump's dispute with Brazil doesn't serve US interests and isn't permitted by US law, so he's not representing the US when he pursues it, even if he's illegally abusing the powers of his office to do so.

It's a subtle distinction, of course, and I don't blame anyone outside the US who decides they're not going to bother making it.