If a genuine mistake i dont think it takes away from what else they have said especially when it is from personal conversations the have had.
I'm not going to discredit everything they have said due to what simply could have been a misunderstanding.
Neither am I, but an objective judgment has to prioritise demonstrable facts over hearsay.
Essentially, they are trying to present a narrative that ignores that their son was never going to be titled prince, and tries to present it as some form of vindictiveness. Rejecting this outright is the only logical course of action because it's demonstrably false. Even if the claimed conversations took place, which is the hearsay, they aren't the reason for the decision, and the attempt to present it as such is fundamentally dishonest.