Simply put, when it says *it* is in the writings of the earlier prophets, it may be referring to the teachings of the Qur'an, a reference to the Qur'an, etc.
When we say something is *in* something else do we mean it is literally and completely in it?
"YOU are *in* trouble!"
Now do I mean YOU are somehow interrelated and a part of the concept trouble? Or do I mean the word "YOU" is a subset of the word "trouble"? Or is the meaning of "YOU" the same as "trouble"? Or have I told you *how* YOU are in trouble? No. Well in a similar manner, the author of the contradiction should not assume "it is in" means "it is". That is assuming one particular understanding of *how* it is in the writings of earlier prophets. A contradiction is not proven again. All praise is due to God.
Do we throw out the verse as illogical (that is what the contradiction's author proposes) or do we throw out the author(of the contradiction)'s analysis of the verse as illogical?