I get it, you thought you had something and you were going to 'own the libs' with it. You're probably mad about it but really you're just embarassed.
But imagine if you'd taken the time to learn the definition of African American and then spent some time learning about the culture and history of South Africa. You could have learned about the different waves of colonization. You could have learned different names refer to different people that often carry specific encoded meaning beyond the most facile definition you went with.
But you know what? Take heart, because now you do know better. So, the next time the racist Uncle you heard this from repeats it, you can say, "Hey, Uncle Bob, I learned something the other day that's really interesting. People of Dutch ancestry born in South Africa are Afrikaners, so Elon is actually an Afrikaans-American. And African American specifically refers to Black Americans who are descended from west or coastal Africans who were enslaved. Which means, as VP Harris's ancestors were coastal Africans who were enslaved in Jamaica AND she is an American, she is African American. You know what, as I am talking about it, it feels kinda racist how we're constantly trying to diminish her by insisting that she's not Black or. We should stop."