Relaxjack, why would the disbelievers say: "These are nothing but tales of the ancients" if the tales of the ancients were not available to Mohammed?
You would only accuse someone of something if it was actually possible. No one would make such accusations if the stories were not known/available during that period. It's like me accusing somebody of causing death by driving before the car was invented. This would be ridiculous and not even worth a response but of course no seriously would say this. The fact that these accusations are made and the Koran feels the need to respond to these accusations seems significant.