She asked if it was a question of law or of fact. Given those two choices, it's an issue of law. That the proposition stated can't actually be proven as a fact doesn't change anything.

Is there any way in the law to treat statements of fact that are not provable? Suppose a case hinged on whether string theory were true. This is a question of fact -- string theory is either true or not -- but as it's not provable, how should the law treat it?