This doesn't seem convincing to me, Indigenous Australian populations maxed out around 1 million people, across the entire island. Considering the population density and ranges of these creatures, I don't see how they could physically do so.
> new (less mgafauna friendly) cultures emerged
It's important to remember Australia's isolation here and the cultures of Indigenous Australia. All Indigenous Australians were nomadic hunter gatherers, with a deep seated animalistic dreamtime belief system, it doesn't make sense that a new culture suddenly rose up and caused mass extinctions, as all of these cultures remained within their land ranges, and without evidence of widescale war in Indigenous Australia, I don't see how this would happen.
>or subsequent migrations caused one of these?
Not really, again 20,000 years and Australia is a big, big place. Like really big, and most of these creatures had wide ranges. I don't find the human hypothesis has enough stock to stand up under scrutiny, especially considering these extinctions happened at the end of an ice age.