"The vegetation had gone from being dominated by plants that preferred a drier environment to water-loving aquatic species. "This doesn't occur naturally,"
Um, every swamp on earth has a man made origin? Ridiculous. It is perfectly possible the swamp came first, the eels second, the people using them third. The core procedure described only detected swamp plants 8000 years ago, not the whole economy. "But in recent times they dig things that produce additional swamp". If so, because they were running out of swamp. Doesn't mean they made all the swamp in existence 8000 years ago. Get a grip.
As for the statistical analysis of stones, um, nowhere is it written that all geological processes produce a normal distribution of stone sizes. There are many places where you find naturally occurring rings of rock, rocks in particular size ranges, etc, for reasons like thawing and cracking. Is it possible they were huts? Sure. Is it possible the fellow ridiculing the idea simply knew more geology and there isn't any reason to think the circles man made? Also sure.
Then there are the details of the hierarchical society and the four wives per chief and all the rest of that. Without the slightest bit of evidence. Not one scrap is mentioned, and a moment's thought is all that is necessary to see there cannot possibly be any evidence of such social arrangements and systems back 8000 years in the presence of swamp plants, or in rock huts if they are huts (and if so, where are the associated accumulations of bone etc?)
Then there is the crazy idea that it was all still there and thriving before the nasty white people came, and nobody noticed it. For which there is no evidence or testimony. The only thing they actually know is that people there eat eels and have farmed them for a while. They have no idea whether the system supported many people (that it could doesn't mean it does - North Korea farms rice). They have no reason to think if it did, it all didn't collapse for some internal reason long before anyone pale arrived.
Then there are the supposedly useless sticks from elsewhere. Um, nobody collects useless artifacts from the four corners of the earth, least of all people in poverty. They are almost certainly weapons - "spearlike". And they are evidence not of elaborate systems of trade - why would anyone think eel meat would keep over hundreds of miles of travel? They are instead evidence that at least some of the people who lived or passed through the area were nomadic. As just about the whole aborigine population was and still is.
What is pretty obviously going on here, is that anthropologists and other slightly more scientific types are inventing fantasy social systems for aborigines and projecting them backward deep into the past on the thinnest sorts of evidence. Is any of it possible? Sure, lots of things are possible. Is any of it established by old swamp plant life and recent eel meat leftovers? Not remotely.