I don't think there's a "guarantee" that it wasn't backdated, but there isn't any evidence of that either.
That's how evidence works for extraordinary claims. Alas, scientific rigor is a harsh master.
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.