I don't know how a fact can be 'indisputable'. All one as to is say 'no it isn't!'.
Pure genius that link is.
Gravity (for example) is considered a fact because it is empirically verifiable and generally considered indisputable.
Fact (and by logical extension Truth) is quite rare, but not non-existent.
Without some tacit agreement on at least a few fundamental facts (like language), communication between two persons would be impossible.
And if you can't communicate with someone, it is impossible to confirm if you disagree with them.
Facts exist as a logical necessity, but not everything (and actually a great many things) that people consider facts are merely opinions.
If you merely "gainsay" the law of gravity, you are likely to be dismissed as either disingenuous or insane.
The battle lines are drawn exactly at the place where you disqualify your opponent.