When a matter occurs in the Gita, then there is no higher certificate needed!
If we question whether the matter of sannyAsi having the only right for shravaNa etc. has occurred in the teachings of the Lord, the answer is yes!
“All karmas finally end up in jnAna” (IV – 33, 34), says the Lord and continues “The seers of Truth will teach you the jnAna. One should bow to them, be in servitude to them, and learn by questioning and further questioning”.
Ending up of karmas means thereafter it is only sannyAsa. Does it not then mean that “Only such a sannyAsi has the eligibility to receive the teaching of jnAna”?