I don’t live in South Africa.

Then you lack the context to understand why the advert is "incredibly racist". Schools and various organizations/companies are still trying to force people to chemically straighten their hair despite many protests against it over the past few years. Given all this very recent historical context, the ad is racist, regardless of the intentions of those who made it.

Historical context:
https://www.timeslive.co.za/news/south-africa/2020-06-18-pupils-at-pretoria-high-school-for-girls-stage-protest-against-racism/

https://www.iol.co.za/pretoria-news/learners-at-st-marys-diocesan-school-for-girls-demand-change-49960544