It’s a historical fact. How do you prove there was once a man George Washington? Or it that untrue without proof?
So is Evolution :-), dinosaurs, lice growing from Eggs, or many other “unacceptable” Hashkafas.
I can prove there was a George Washington (for example, he signed the constitution, which is still available for viewing today). Can you prove anything?
Saying “It is a fact” is equal to saying “I have no proof”.