We don't know whether there's any means of verifying the story until people actually put some effort into verifying the story. Also, it's not unprovable. Perhaps in a mathematical sense, but that's not even required in a courtroom.
Consider all that's been written about Altman's character, using such weak signals as "this former co-worker said this about him". Meanwhile there's this disturbing piece of information that could be a strong signal that is completely ignored.
Have you spoken to anyone in your life who has been molested, or abused? It really cuts through the abstract arguments and illustrates that this dynamic is how abusers get away with it.