A person may acquire certainty about a prophetic statement through direct hearing or other personally conclusive circumstances, even where a later mass transmission is not involved. A companion's immediate audition is the clearest example.
This does not invalidate tawātur. It shows that mass transmission is not the only conceivable route by which a particular person can know the provenance of a statement. Some routes are public; others are restricted to an eyewitness or participant.
Understanding the sounds and understanding their precise meaning are further distinct tasks. Hearing authentic words does not guarantee infallible interpretation.