Look, that’s outlandish to hang your position on that. Maybe they only have one fossil from Antarctica because almost all of it is under a mile-thick sheet of ice! And Australia itself has a terrible fossil record at this time. Heck so did S. America for that matter. The authors of your link say so themselves…
However, the poor fossil record from South America, Antarctica, and Australia does not exclude that Djarthia, like Dromiciops, could be of South American origin and had a pan-Gondwanan distribution. Additional fossils from Australia or South America will shed more light on the early Australidelphian relationships and their biogeography in mammals.
So when you write…
I agree there was radiation in Australia, the question is “how much”? T the graphic is showing one view of the evidence, does not include Antarctica, and the amplifying text of the study does not support your assertion. I do think it was a top-drawer study. They are to be congratulated for meticulous research, but it doesn’t show what you want it to show. The evidence is inconclusive.
But it does not show how diverse that branch was when it first came to Australia. That’s my point. Until we know starting conditions we can’t measure the power of radiation. The study itself says…
The relationship among the four Australasian orders is not resolved, and of special interest is the phylogenetic position of the marsupial mole,…
So while they can say that all Australian orders are more closely related to each other than to Microbetheria, they can’t figure out a tree within the four orders themselves. This would be the case if there was no tree, if they were already separate when they came to Australia. No one of them would be any more closely related to any one group than the other. BTW I believe this is the case among Placental mammals and marsupials- but I am having a bit of a rotten day and don’t wish to get into that now.