The source notes the role of compatibility (عدم تنافي) and incompatibility when relative restriction limits a subject to an attribute. In ifrād, the listener imagines two qualities together, so the compatibility of those qualities matters in framing that mistaken combination.
Qalb overturns a contrary conviction and naturally involves clear competing determinations. Taʿyīn resolves uncertainty and has a broader range, since its primary purpose is to identify one of the alternatives.
The rule is not a mechanical claim that two words are always opposites in a dictionary. What matters is how the alternatives are understood and contrasted within this particular exchange.