There is a basic misconception here: the one that "real" characters work like robots or purely mathematicallay driven virtual entities. Ciri and Geralt were "real" characters in the books. A "real" fight can lead to many outcomes. Even the best sword fighter might die in a simple fight, no matter if theoretically(!!!) nobody should be able to beat him. Real characters make mistakes. Real characters do ingenious or stupid things. Real characters have emotions that might lead to victory or defeat no matter the skill factor.
So no, if Ciri was able to kill Bonhart then there is no way to say for sure that Geralt would have beaten him as well in a real fight. That's just not how "real" characters in a "real" situation work.
And I might remember you that the world's best sword fighter died at the hand of a poor peasants without any weapon skill at all at the end of the books. That completely destroys your theory. Not even to mention that Vilgefortz, the world's most powerful person, was killed by the hand of a simple Witcher, not because he had more skill but because Vilgefortz made a crucial mistake in their last encounter - and Geralt had the element of surprise on his side. It's the little things that can radically change the outcome of a real encounter.
So give me a fucking break, yeah. Don't make up arguments based on the misconception that people were robots...