That's why they are effective. I review Community Notes sometimes and the right assessment is almost always "no note needed". A lot of attempted CNs are just arguing with the poster's opinion, which belongs in replies. CN is meant to be for correcting cases where something is objectively false or missing critical context, and it does quite well at that. People are very good at spotting edited videos, mis-dated photos and so on, which is the bread and butter of real fact checking. Not very exciting but useful. Facebook could do worse than just reimplementing the system. It's certainly far better than letting activist run NGOs be editors.
I don’t think there’s any way to algorithm your way out of non-trivial fact-checking. Tech is not the solution to these kinds of fundamentally social problems.
(I should add that the best-case scenario here is an emergent and stable cabal of intellectually-rigorous editors, perhaps of varying political persuasions, similar to what happened to Wikipedia. But that’s barely different from fact-checking by some NGO.)