It only works if you check claims with facts. Known facts. I'd like to think that's the very definition of what a "fact" is. At CNN, apparently, they have a different definition. To them, a claim is "false" if they've never heard any facts to the contrary.

CNN’s lead fact-checking journalist said President Donald Trump was “marginally more careful with the facts” than usual in his address to Congress on Tuesday, but still gave “an extremely dishonest speech.