لَيْسَ is used for negation. بِوَجْهِ كَذَّابٍ denies that the face suggested habitual falsehood. كَذَّابٍ is an intensive expression for someone who lies repeatedly.
تَبَيَّنْتُ is a past-tense verb; the final تُ identifies the speaker as “I.” This helps the learner recognize that the statement records the companion’s personal perception.
The moral lesson is broader than appearance: the companion was moved by the Prophet’s ﷺ character and truthful message, and then reported the practical guidance he heard.