Lacuna opens with an extremely peculiar author’s note. Fiona Snyckers informs the reader that her book is not a retelling of Disgrace (1999) by the Nobel prize-winning South African novelist JM Coetzee, but that it does have an “intertextual relationship” with that harrowing, controversial and much-garlanded novel. Lacuna will feature a character called John Coetzee who is “entirely fictional” and another called Lucy Lurie who, like her namesake in Disgrace, is the white victim of gang rape by black men but is otherwise “original and fictional”.

“I use the character of Lucy to explore the phenomenon of white feminism in South Africa,” she announces. For Lucy is “trapped in her own racism and unconscious biases”. She is “solipsistic and selfish”. She makes “flawed life choices” and “practises a shallow form of feminism that does not take into account intersectionality”.

Can you imagine if all authors framed their books in this way? “Anna Karenina will make some bad choices in this book.” “Captain Ahab acts out a damaging form of masculinity that does not take into account the feelings of the whale.” It may be that such prescriptions are more common in young adult fiction, where Snyckers made her name, but her tone reminded me more of a Soviet compendium of English writers that my husband once brought back from Russia, which introduces Wilde, Waugh and Wells with stern warnings about how a good communist is expected to interpret them. Only: what are we to make of her insistence that this victim of gang rape is “solipsistic and selfish”? Is this some kind of postmodern joke? I don’t know.

Despite the fact that Lacuna is more heavily signposted than the M25, I’m still not entirely sure what it’s trying to say. It purports to be a feminist response to Coetzee’s novel, but it muddies the waters by rewriting the story of Disgrace and its invention in several fundamental ways.

This version takes place in the present day rather than the more immediately post-apartheid setting of Disgrace. Lucy Lurie is not an earthy lesbian collectivist but a heterosexual junior academic who teaches in the same university department as John Coetzee in Cape Town. The real-life Coetzee had already written several acclaimed novels by the time he wrote Disgrace. Here, Coetzee is a sexist old fart approaching academic obscurity. However, when he hears of his young colleague’s brutal rape at her father’s farm, inspiration strikes and, to everyone’s surprise, he produces an international bestseller.

The novel opens two years after the attack that made Lucy, 28, “South Africa’s number-one rape victim”. She is furious that Coetzee treated her trauma as a metaphor for the overthrow of white supremacy in South Africa and becomes intent on confronting him about it. She is similarly outraged that Coetzee chose not to describe the rape scene in his novel, leaving instead a “lacuna” – a space unfilled by women’s voices. “I am nobody’s lacuna,” she insists.

Snyckers introduces a number of discrepancies. Here, Lucy is raped by six men as opposed to three (was three not enough?). She takes the morning after pill whereas the Lucy in Disgrace decides against terminating her pregnancy after the attack. And while her father, David, is an ex-academic who was dismissed from his position because of sexual misconduct, in this version it was his farm rather than Lucy’s that burned down, leading him to become fixated on the insurance claim.

The plot twist, when it arrives, is not in itself ridiculous but the execution is, mainly because Snyckers is too caught up in her political messaging to explore the emotional implications. She seems to imagine that giving voice to a rape victim means reducing all the other characters to crude antagonists. Her father becomes evil; her best friend, Moira, is a compendium of every insensitive thing you could say to make a traumatised rape victim snap out of it (“Don’t you ever get horny, Lucy?”).

And the beef with Coetzee is fundamentally misguided. Her misreading centres on the false idea that the rape in Disgrace functions entirely as a convenient post-apartheid metaphor and that Lucy’s decision to keep the child is a fantasy on Coetzee’s part about a happy mixed-race South African future unencumbered by history.

The original novel is subtler, bleaker, more ambivalent. Any sense of redemption for the characters is mixed up with ruin. You can interpret Lucy’s decision to keep the child in a number of different ways: as a radical act of forgiveness, as a weird form of white reparation, as a way of punishing her father or even simply as an assertion of the agency that Snyckers seems convinced that she lacks. And it’s not as if the Lucy in Disgrace is lacking in self-awareness. “You behave as if everything I do is part of the story of your life,” Lucy tells her father at one point. “You are the main character, I am a minor character who doesn’t make an appearance until halfway through.” It’s Coetzee who invites the feminist retelling.

There’s another discrepancy that’s hard to account for too. Despite Snycker’s fondness for terms such as “intersectionality” and “white fragility”, and her concern about female voices being erased, she has completely written out the character of Melanie Isaacs, the student who is coerced (and arguably, on one occasion, raped) by David Lurie. In Disgrace, she is referred to as “the dark one” and many critics have interpreted that as meaning she is black or mixed race. This is the event that gives the book its title.

Perhaps it would have helped if Coetzee had spelled all this out in big capital letters at the beginning.

• Lacuna by Fiona Snyckers is published by Europa Editions (£12.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply