Extraordinary claims require extraordinary evidence.
The tree sisters story [1] (which was actually taught in schools) is apparently completely fake and the modern highway network apparently resembles the way aboriginals navigated Australia too [2]
[1] https://en.wikipedia.org/wiki/Three_Sisters_(Australia)#Abor...
[2] https://theconversation.com/how-ancient-aboriginal-star-maps...
Aboriginal people are just people and they don't have some magically ability that allows information to be passed down via oral transmission through 300 generations.
Edit. The huge diversity of Aboriginal languages is proof that information was not transmitted accurately. If you can't even preserve the word for "foot" (just an example) down 300 generations how can it be expected that a story of even less practical utility and far greater complexity would be maintained accurately. The whole idea is ludicrous.
I could see where this kind of cultural and physical mixing (migration) could dilute or mutate folk tales, washing out distinctive 'fact-derived' aspects of the stories
The Basques have been where there are for a long time and they have had a oral culture for most of the time and yet their myths are not any more factual than anyone else's.
Without debating whether the information preserved is factually accurate, it's known that oral cultures develop interesting and complex -- from an information-theoretical perspective -- mechanisms for preserving information, and for making it possible to retain accurately in memory amounts of information that tend to shock people from cultures oriented around writing as the means of information preservation.
If your cultures can't preserve basic words, what possible mechanism could preserve useless facts in complex stories who's purpose is purely to entertain around the campfire?
And if you don't think the old oral epics have value -- you seem not to -- then I don't know that it's really worth discussing them with you.
Their epics just weren't intended for that purpose. You might as well criticize modern literate America for the vast quantities of fact-free written material being churned out.
However there is an odd problem. There is evidence that literacy is in direct conflict with memorizing complex oral traditions. Oral traditions can be surprisingly long - for a well-known example consider that Homer's works were preserved as an oral tradition. People who can read and write are unable to memorize works of this length. And anthropologists learned the hard way that teaching someone who has already memorized things of this length causes them to forget what they previously knew.
Most oral epics and stories were never intended to accurately transmit facts about the past. The idea of history as based on facts is a very recent idea and it is one of the reasons we have so many problems with the written records from ancient historians. They had no problem making up entire stores and conversations that never happened as long as it furthered the purpose of their writing.
The purpose of these oral traditions is not to transmit facts, but to explain to the next generation what is important to their culture and/or entertain.