Layman reader.
The thesis is in the epilogue, which mainly boils down to the author talking about the nuances around academic history versus narratives presented as part of Aboriginal rights movement and that maybe there is no need to panic if people tell different histories about places and events for different reasons. His actual stance though is that Aboriginal history is richer and more complex than the usual narrative along the lines "they lived happily before colonisation then Europeans came and messed everything up, on purpose and by accident, and the Aboriginal people are perfect victims".
The preceding chapters lay out the case of a specific place and time, looking at one particular people. As one might expect, on top of the horrific statistics of disease wiping out horrendous numbers of people, the rest is more complex. There was a variety of opinion among settlers, depending on their background and interests -- ranging from "well duh they are stealing your sheep, you scared all the game away and messed up the edible plants" to "kill them all" (interesting that a chunk of settlere were veterans from British colonial wars in other locations. Not surprised that came down badly).
Same with the Djadja Wurrung - some tried to assimilate and adapt, some tried to exist in two worlds, some stuck to tradition, all with varying results. Like you would expect.
It's also interesting to trace the history of reservations back to something well-intentioned and positive and something that in its original form is not dissimilar to modern reconciliation efforts granting land and funds - although more hamfisted and executed with mixed success by people not always well-suited to their position. Empire had HR issues. (But then it devolved into assimilationist paternalism, so).