Theme as Introduction

I've read much advice suggesting that the opening page of a novel should introduce the novel's key themes (and that each chapter's opening should do the same thing, but more specific to the chapter's themes).
Here's Kate Grenville's introduction to Sydney from her Man-Booker short-listed The Secret River, which I'm currently reading with enormous admiration:

'One hot afternoon in the January of 1788, with big white birds screeching from the trees by the shore, a captain of the Royal Navy had sailed into that body of water and chosen a cove with a stream of fresh water and fingernail of beach.'

Imagine all of the ways she could introduce this new land.
Why did she choose these descriptions?
I can only guess.
Hot afternoon? The British convicts will have to acclimatize to this new land. Thus far, we have had freezing London winters that claim many lives. Kate is opening with a kinaesthetic description.
The big white birds screeching? Now we have sounds too: Kate is giving us the essential experience of this discovery. We have an undiscovered place, ruled by nature; screeching is a harsh word and a harsh sound, conveying the might and, perhaps, hostility of nature. And big, rather than small or meek or welcoming.
Trees by the shore? Again, a new land in which the trees have yet to be chopped into building materials and firewood.
Stream of fresh water? Same reason (it is unpolluted by mankind), and we can see how this might be a suitable place to build a settlement.
In this opening, we have mention of a body and a fingernail, both of which augment the kinaesthetic nature of the prose. (Indeed, Kate makes much out of body parts, especially fingers and fingernails which she repeats throughout this chapter. She also mirrors these descriptions in nature, writing of paws and talons.)
So we find nature, undisturbed and powerful, and a man sailing into its midst, and the reader is invited to feel and hear and see this place.
Reading on, Kate continues to pit humans against nature: she composes page after page of descriptions of rocks and rapids and heat and poisonous spiders, and of settlers battling their environment, eating into the continent and planting crops and felling trees.
This, I would suggest, is what Kate wants us to know, and is how she chose to construct her introduction to this terra nova: the fluctuating balance between conflict and harmony in which humans and nature reverse roles as protagonist and antagonist from page to page.
Even the novel's very title alludes to this theme: A river; it is secret; it is discovered.
I won't know what it does next until I read on.