The Boers have lived in Africa since the mid-17th Century. After that much time, what are they if not African?
A lot of Boers descend from very early European immigrants to then-Dutch South Africa (mostly Dutch, but some Huguenots, Germans, etc. too). Many of today's white Americans came over well after American independence, although many of today's black Americans had their ancestors dragged in chains here long before. I don't think it's very relevant though to this subject.
So because only my great grandparents were born here means that I can't claim to be really American despite being born here and desiring to live in no other country and likely going to die here.