I don't think there's a "guarantee" that it wasn't backdated, but there isn't any evidence of that either.

Since backdating would be one of the ways to fake this, the onus lies in those that claim the prediction was legit.

That's how evidence works for extraordinary claims. Alas, scientific rigor is a harsh master.

"extraordinary claims/extraordinary evidence" isn't scientific rigor, it's adding qualifiers to make the evaluation of the claim and evidence more subjective.

It's literally how the scientific method works.

There's a reason RV is considered pseudoscience.

It speaks volumes you don't see any problem with the lack of proof that the documents weren't backdated, and get defensive when people tell you this must absolutely be ruled out if we're ever to even start considering your alleged evidence.