The qualification fi zaman al-waḥy in the ruling of mufassar is deliberate. Naskh requires a subsequent authoritative revelation; a scholar's later opinion cannot abrogate an earlier revealed command.
After revelation ceased, no new abrogation could occur. Thus the source's reference to possible naskh for mufassar must not be read as authorizing anyone today to cancel a definitive text by personal decision.
Muhkam has the further distinction of being intrinsically non-abrogable, even when considering the theoretical possibilities that existed during the period of revelation.