Not sure of that last claim, as Australian natives are (AFAIK) considered one of the very oldest groups to separate from other Homo sapiens. IIRC, they're the only major group that has no Neanderthal DNA, because they migrated/were separated before H. sapiens met H. neanderthalis.

There are lineages in Africa that split from other lineages in Africa before the Australian aborigine split. Add on more frequent (but still rare) mixing for even more diversity. Mixing makes the majority more homogeneous but can increase diversity at the extremes.