Stephane - "They are the ones making the claim, they should be the ones to prove it."
It's kinda funny, actually.
They're in a very weird place where - from their POV (due to 100-plus-years of ideological policy and internal stated standards) - they have to believe it...
...but...
...on a fundamental level, they don't dare try to really and truly prove it, because...
a) ...any such proof ends up being so much conjecture, supposition, and unfounded assumption that it's pretty much nonexistent, and...
b) ...virtually all authentic, demonstrable evidence (based on history, science and logic) almost always ends up disproving the claim.