You are incorrect. The logic behind a separate incest exception is that the vast majority of incest is at least coercive/groomed if not forcible rape, but qualifying for the rape exception would require meeting the significant and probably-not-achievable standard of proving rape months after it happened. And in many incest cases that means specifically "proving rape" after years and years "intimate relations," given again that most incest cases are coerced/groomed for years or decades.

Contrast with the simple prenatal paternity test.