User Obie2.0 addresses your literal question as to whether the author has been questioned on the topic and I don't have anything to add on that. I looked to see what I could find about the author generally, to see if there were clues there.
What I found was that there is little detailed biographical information about Sally Andrews online, at least in English, but that in interviews she often makes reference to her past as an activist, both on environmental topics and against South Africa's apartheid regime. The most detailed such reference I found was an interview on the Mysteryreadersinc Blog, where Andrews says:
I was an activist in the height of the anti-apartheid struggle. Comrades and friends were killed, tortured and arrested daily. There were taps on our phones and spies in our organisations. I was threatened and angry and fighting. We were at war, and I had no patience for ‘the enemy’: racism, sexism, apartheid, capitalism, and anyone who was tainted by them. There was little softness in my fists, or in my heart.
At a book launch event she also said:
To be honest, when I examine my motives for this book, I do have some intentions of how I hope to affect readers. I am not sure if I would define them as political, emotional of spiritual intentions, but they are there between the lines. I feel that if intentions are overstated then they can be less effective. I’d rather they are evoked from the reader, as Peter van Straaten said opening an art exhibition recently: it’s not for the artist to tell the viewer what to think or feel about the art, the viewer, or in this case the reader, must discover for themselves what the writing means to them.
My take from these statements is that it is improbably that Andrew intentionally engaged in racial stereotyping, but that that is a reading that is available to people.
EDIT: Note that this section has been added after the answer was accepted.
There has been some discussion in comments, some now removed, as to whether Andrew's claim to activism are credible. I am aware of no specific grounds to suspect her of embellishment and am not sure where the impetus for such distrust arises. However, it gave me the push to look further to see if I could find anything corroborative.
Web search result are dominated by references to her written output, which includes a publication called 'The Fire Dogs of Climate Change' which is a call to action and includes some personal anecdote. Again details are lacking on activism against apartheid, but Andrews gives her parents names, Paul and Bosky Andrew. Searching on those names leads to the identification of her father as an architect and activist. His obituary in The South African Sunday Times says:
Typically, he was interested in practical solutions based on extensive research and solid technical arguments that could be implemented and change people's lives.
Apart from achieving results, this was in large part why, although his phones were constantly tapped, he managed to avoid being arrested.
He certainly took chances, however.
He and his wife, Bosky, who lay down in front of the bulldozers at Crossroads while he concentrated on more technical solutions, hid members of the ANC underground who they'd got to know in Lusaka, at their home in Constantia.
While this does not constitute proof of Andrew's own activism, it does place her in a family setting where activism was a feature and makes it less likely that she makes up her activist history from whole cloth.