But the Cape Dutch and the Boers shared the same ancestors, didn’t they?
To a limited extent. The VOC dumped a lot of different nationalities at the Cape. The main groups the VOC brought being: German / Frisian / Danish / French Huguenot & Dutch. There was a bifurcation soon after the establishment of the Cape colony though. The poorest ones began to trek out of the society as “they could not cope in Colonial society” & chaffed the most under the autocratic VOC rule. There was a long wave of German immigrants to the Cape & when many of the ones who arrived in the 1700s - they settled directly on the Cape frontier where the Trekboers / Boers had emerged. Thus the Boers have definitely quite a bit more German roots than the Cape Dutch.