Interesting point. I've read through the thread that you mentioned, and the key point seems to be on the third page: "the sentence is correct as long as one of the two interpretations (or structures) makes sense, supposing no other grammatical or meaning errors exist". This is what we might call a "principle of charity". Remember that, although we're looking for problems in answer choices, we're not trying to
make them wrong.
If we analyze the above structure as
not X but Y, then answer C seems to be the more parallel one. However, if we analyze the structure as
X but Y, then answer D is correctly parallel. Obviously only one of the answers can be correct, so we have to look for another difference.
Ron makes a good point that:
"Are" is also necessary in that sentence because of "simply".
"Simply" is an adverb that describes the fact that these things are random byproducts; i.e., it describes the verb "are". If you remove "are", then "simply" is an orphan with nothing to describe.
Think of the following sentences:
1
A shark is not a mammal but a fish. (This is fine according to the structure "A shark not X but Y".)
2
A shark is not a mammal but actually a fish. (This hopefully sounds funny - that's because the adverb 'actually' doesn't have a verb to modify.)
3
A shark is not a mammal but is actually a fish. (This is fine because it follows the structure "A shark X but Y".)
Now I know what's coming next, you're going to ask if 4
A shark is not a mammal but is a fish. is okay, or whether the word 'is' is redundant. I'd say hesitantly that it's okay, and that I haven't seen a GMAT problem that relies on choosing between examples 4 and 1.
Finally, I should note that this problem is more that 10 years old. As much as it's great to look at GMAT Prep resources, please remember that GMAT does change over time, and this problem may be less relevant than more recent ones.