The last ice age ended about 9200 years before Plato was writing. He was 2500 years closer to events than we are, and therefore about 21% better informed, I suppose.
Humans are story tellers. Stories last a long time. The youngest fossils of Megalania, which grew up to 7 meters (23 feet) in length and weighing over 600 kg (1,300 lbs), which is the largest terrestrial lizard known to have ever lived, date to around 50,000 years ago, confirming that it was alive when the first Aboriginal people arrived in Australia. Early human inhabitants and the giant lizards would have shared the same environment, and Aboriginal oral traditions about giant goannas may be based on these ancient encounters.
Those stories lasted 50k years.
Do you have a suspect for the Bunyip, by the way? I like that one.