Put the backstory into the mouth of a character, rather than narration.
If I opened, say, a mystery story, and it began, "Fred Smith murdered his brother John," I would understand that to mean that that is what actually happened. If later in the story I read that in fact Fred didn't kill John, I'd be turning back to that page and saying, "Wait, but back here you told me he did. What is this?"
Likewise, weasel words in a narration are a giveaway. If you wrote, "Everyone believed that Fred Smith murdered his brother John", I'd understand that to mean that he didn't, but everyone thought he did.
But the problem completely goes away if you have a character say it. If in a mystery story I read, "The detective said, 'Fred Smith murdered his brother John'", that might or might not turn out to be true.
I expect the narration of a story to be true -- "true" within the story's world of course, not necessarily in rea life. So if the narration says that a character said X, I understand that to mean that he really did say X. But I readily accept that he may have been lying or mistaken when he said X.
Note this only applies to a third-person narrator. With a first-person narrator, I understand the narration to be what that person knows or believes, which may or may not be true.
Some stories stretch this point by having hallucinations or dreams or the like described as if they were real, and then saying, no, that was a dream. In a story about a character who cannot distinguish his hallucinations from reality, this can be effective. Done well it can be effective. Done poorly it's lame and readers will get annoyed that they never know what's supposed to be real and what's supposed to be an illusion.
(I'm suddenly reminded of an episode of Buffy the Vampire Slayer -- a classic of Western fiction that will no doubt go down in history with Shakespeare -- where Buffy's mother collapses, Buffy calls an ambulance, the paramedics revive her mother, you see her with her mother in the hospital ... and suddenly the scene snaps and her mother is lying dead on the living room floor and you realize that was all her pleasant delusion. My daughter said she thought it was obvious and lame. I thought it was very effective. So it's reader's point of view.)