A companion could directly hear the Prophet's words, while later generations must establish the same wording or meaning by reliable transmission. Hearing, authentic attribution and the truth of the message are distinct stages.
A false attribution does not become true simply because the Prophet himself is truthful. One must first establish that he actually said what is attributed to him.
In a mass-transmitted report, certainty about attribution may arise through transmission; its authoritative content is then connected to the separately established truthfulness of the messenger.