Sounds like circular reasoning.It is correct that OT books were written many centuries after their original writers does not mean that they are fiction. They might not be revealed word of God, but they do contain words of God.
Quran confirms many things for OT and NT books and those can be treated as authentic.
The QUran tells us it is the word of Allah. The word of Allah is perfect. Therefore the Quran is perfect. There the Quran is the word of ALlah. And so on.
A implies B which implies A, etc.