The subtle difference is paramount to understand.
The Arabic (language) of the Qur’ān is dependent on certain external sources. It is only a dependency as far as the meaning of a word is concerned.
Qur’ān is not a book of language, rather it is book of ideas, concepts, philosophy, instruction etc. These constructs are dependent on the context of the words used. The concept is its own. It expresses religion by using the language of the Quraish, this doesn’t mean that it also depends on the constructs or concepts of the Quraish, that they arrive at, with the use of words of Arabic.
Like wise, the Ahadith are used to learn a language and they don’t carry an independent source of religion in them. When we say that the Qur’ān is indirectly dependent on the Ahadith, what we are trying to say is that the Qur’ān needs the Ahadith to express is constructs. Indirectly, trying to show that Qur’ān cannot be understood without the concepts derived from Ahadith.
What Farahi school of understanding is saying is that the concepts of the Qur’ān come from Allah although they are expressed in the language of the Jahiliya period. The dependency is on the words and their meanings not on the concept or construct derived from the organization of those words in a sentence.
As I am writing this in English, I am expressing a philosophy, an understanding, although I am dependent on the words of the language but I am not dependent on Shakespeare’s philosophy or concept.
I hope it is clear by now.