To be fair oral histories are borderline valueless without multiple forms of corroborating evidence to support them.
Oral histories inevitably become something akin to an intergenerational game of Chinese whispers that continues for millennia, with each member in the chain making their own embellishments to the story. They tend to effectively evolve into and/or merge with mythologies, so you can imagine how little value most of those stories have when it comes to deciphering the truth.
In saying that, there's plenty of historical evidence that Indigenous ball games, including Marn Grook, existed. There are first hand accounts, contemporary paintings and even some later photos of people playing them (such as the one featured in his article), other physical evidence, etc, so to suggest that they're a myth that didn't exist is pretty insane. You're getting into flat earther or dinosaurs didn't exist territory at that point.