You have to remember one thing about creationism, they have abandoned all semblance of need for their explanation to have any relation to reality or evidence.
For that reason it is possible for them to come here and answer your question as follows.
For a brief period of time after the flood the continents were much higher before they sank a little deeper into the crust after runaway plate subduction was finished. This presented a number of land bridges to animals that no longer exist.
Or vice versa
Ocean levels took awhile to settle to their current state post flood which gave an opporunity for animals to cross land bridges.
Or think up anything else crazy off the top of your head
The continents moved to their current position AFTER the flood was over.
All marsupial animals are decendents of one kind of marsupial that used to know how to fly and subesquently lost that ability due to devolution because of the fall
You need to remember, when you don't care about evidence or even basic plausibility; if your only criteria is that whatever explanation should not contradict the Bible, then anything goes!
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)
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