You must have missed that point entirely about how the rain forest had been separated by climate shifts and then rejoined by their expansion 8000 years ago. That separated the frog populations and this article examined the repercussions of the forest coming back together.
No doubt there's plenty of evidence of forest boundaries over the years available.
Of course if you think the earth is only 6000 years old, then I see your problem.
Why would I think the earth is 6,000 years old? Think I just fell off the turnip truck?