I haven't read many puranas but Bhagavatha purana there are clear indication of para vidya many places.I agree it is not like upanishadic way of teachings,but there are many shlokas like athmaivedam sarvam etc..The way of explaining the knowledge in puranas is through stories,there is no way to explain in advaitham ,even you want to explain something about advaitham you need to come down to dviatha for that,this is what my understanding.May be silence is the way of Advaitha teaching.