FTR this wasn't a grammar nit; these two different words in this context have opposite meaning! So, as a former teacher of English as a Second Language, I offered the substitution in order to help make the meaning clear.
How do they have opposite meaning? Can I use "by" to declare A and B as opposites in such a context? (Trying to learn, I never heard the usage of "by" like this)
In the former (as OP typed it), it's grammatically suspect but also seems to imply a missing "created" like I inserted. In that case it'd be ambiguous whether the OP feels requirements are not documents, or perhaps they are documents, just not ones "by a process".
In the latter, which I took to be the intended meaning, OP is saying "requirements are a process, not a document."
The "not X but Y" is grammatical and clear, equivalent to (boolean pseudocode) "Y && !X".
Misguided downvote, imho. (shrug)