Reference on how often "To the best of our knowledge" assertions turn out to be false
To the best of my knowledge, it is unlikely anyone has done the gruelling work of trawling through the literature, looking for such statements, and then doing the detailed work of finding out if the claim was true.
On the other hand, if I state such, you'd have to prove I did have prior knowledge to refute my statement. In itself, that could come close to an accusation of plagiarism in the context you cite. Or at least of sloppy checking of previous work.