Why might a sole pupil’s statement that his teacher is “trustworthy” still be insufficient?

Narrator Anonymity: Wuḥdān, Mubham, Majhūl al-ʿAyn and Mastūr

• Every student is dishonest
• Everyone in the Ṣaḥīḥayn is unknown
• His qualifications as an evaluator and the grounds of his appraisal must be considered
• Authentication and criticism are entirely irrelevant