The "problem" with the repudiation of mass Graves and murders of indigenous children in this book, is the problem of conflating "fact" with "truth", and - most importantly - "fact" with community knowledge as vouchsafed by Native Elders.
Truth is, the Indigenous people suffered harshly under White European colonialism. They were not integrated/assimilated as were defeated European peoples when the Europeans were conquered. They and their cultures were marginalized and made impotent. So, the truth is, they suffered. It is this suffering that is "true". HOW they suffered is less important within an oral history culture than one of the Enlightenment. The details are less important than the overall event.
The Indigenous also rely on the Elders to hold history through storytelling. The lack of Graves etc in White European culture means the tellers are not credible or even worse. An error of an Oxford dean is not personal or socially traumatic, indeed, errors are expected. But challenging the credibility of the Elders is attacking their character and social importance. A pillar is being removed.
But there's also financial and political collateral effects to the claim. the Federal government is/was undertaking serious reparation payments on the basis of indigenous suffering. Questioning this major scandal undercuts the moral responsibility of the current electorate to "make things right".
The Streisand Effect is happening here: the more their leaders push back, the more attention is paid to the subject. And the "truth" gets diminished. Which is a shame.