There are regions with higher populations than that

Can you name one of these regions? From what I've seen 100,000 is the most generous estimate of the total Koala population in Australia, so 28,000 being the highest regional population sounds pretty reasonable given the amount of land that they're dispersed over.

These death figures are based on how much of the land has been consumed by the fires. Like I said before, for most animals when their home environments are destroyed they just die, be it directly or indirectly. Thus if you already have data on how many animals live in a given area, and then that area is quickly destroyed, it's very reasonable to estimate that the majority of the animals that lived in that area are either dead or dying.