[improbable22]

I think the time of divergence in spoken language is much longer than that, maybe 300 years (and with lots of second-language speakers, which speeds it up). And starting I believe from dialects closer to Flemish.

The formal written language was indeed standard Dutch, but was not widely understood. For example (IIRC) around 1900 the dutch reformed church in the Cape partly switched to English, on the grounds that at least this was a real European language, in which people might actually understand the sermon! After that Afrikaans got formalised, e.g. the first official bible translation was 1933.