The backup plan of answer choice A makes perfect sense

Hi there, I chose A because I believed that A weakened this conclusion. However, after reading the explanation of why A is incorrect, I still believe that A weakens. The back up plan written in A's explanation says, "Until recently, intellectual historians didn't have a definition of medieval epistemology. We can define medieval epistemology as the epistemological beliefs medieval epistemologists hold. If we want to know whether medieval epistemology includes a belief, we'll ask whether any medieval epistemologists believe it. If they do, it's part of medieval epistemology. If any believe the opposite, the opposite claim is part of medieval epistemology. But, medieval epistemologists' beliefs depended on their non-epistemological beliefs. So, we maybe shouldn't define medieval epistemology as the epistemological beliefs medieval epistemologists hold." However, I think once we insert answer choice A and continue with the opposite of the conclusion, it makes sense. It says, "But, medieval epistemologists' beliefs depended on their non-epistemological beliefs. So, we maybe shouldn't define medieval epistemology as the epistemological beliefs medieval epistemologists hold." My rationale is that since medieval epistemologists' beliefs depended on their non-epistemological beliefs, it is reasonable that the "epistemological beliefs" of these people—which is the opposite of "non-epistemological beliefs"—should not be used for the purpose of historians. I don't know if I made myself clear. I hope someone can help clarify my confusion. Thank you so much!