That they, including ornithopod species, survived freezing winters—as species’—suggests migration became the survivors’ evolutionary countermeasure.
The article also mentions finding similar age bird presence. So perhaps these species were already migratory and wintered farther north in what became Australia, and only traveled that far south as the climate and light became more suitable. Also they probably all had some kind of feathers.
As to how the mud prints survived, perhaps it was an unusually high tide. Perhaps they froze overnight and were dessicated by wind as they slowly thawed. I have old items in my freezer that have dried out—called freeze dried. The article did say Spring.
Do we really know they survived freezing winters?
There is a theory that periodically the Earth’s crust has slipped as the planet spun, and the ice-heavy Arctic and Antarctic began to wobble, pulling those heavier parts of the crust to the equator, and whatever is in the equator, in two places, getting pushed up to the Arctic and Antarctic.
There was some book on that, looking at stuff like this which indicated the sudden switch, which the CIA had grabbed up and archived, which led many to think the Agency lent credence to it. It sounded cataclysmic, because the oceans were slopped around and pushed over some land, washing it all clean with like 100ft walls of water.