This is fallacious statement - nothing is ever 100% accurate 100% of the time, that's impossible.
This is called absolute inaction in ethics - if you say that you can't take action unless there's utter most certainty and there's nothing in the universe that is utter most certain -> you can't take any actions. You're perpetually stuck in indecision.
So it's perfectly possible to reach certainty where someone is practically 100% guilty. Would you say that the killing of Musolini was unjustified? Should he be serving a life sentence instead and we ought to risk revival of nazis just for sake of not breaking this dichotomy? Yeah he's 99.99999% guilty but we can't be 100% sure.