That is a good question!
There's very little information on the actual formation of Afrikaans, both in written and spoken form. The Volksblad
for instance was regarded an Afrikaans newspaper, but seems to be closer to Dutch than modern-day Afrikaans.
The reason for this is that both Cape Dutch and Boer both wrote and read in Dutch(or at least a broken dialect thereof). A prime example would be the "Staten bijbel"(A very popular Dutch bible) which a lot of families owned and which was used to teach children how to read and write. However, both Cape Dutch and Boer spoke different variations of Afrikaans. So Dutch was the written language, but Afrikaans was the spoken language.
I'm only speculating here, but they may very well have used Dutch books. "Die Volksblad" was called "Het Volksblad" so the earlier versions of Afrikaans still had Dutch gender articles. It's absolutely possible to teach out of a Dutch book. My one subject's textbook is full of Dutch quotes from Dutch scientific journals.