Is it possible this rule refers to Buddhist monks/nuns should not openly talk to laypersons about Vinaya, rules of the Sangha (involved word for word repetition)? The practice of Vinaya, being regarded as dhamma (sutta) in the collection, is only for the Sangha’s monks/nuns.
According to the Uposathakkhandhaka (sections 16.8 and 36) in the Pali Vinaya, the Buddha simply does not allow Bhikkhus to recite Patimokkha together with laypeople, non-Bhikkhus, without giving any reasons.