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.

Misguided downvote, imho. (shrug)

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)

"not a (document [created] by a process)"

vs

"not a document but (rather) a process"

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".