To me also is seems obvious that Dhamma (the true, original Dhamma) can be taught only by Ariyas.
Nevertheless, we should feel grateful also to those who just passed on the suttas down the generations without themselves having got to the true, the ‘hidden’ meaning in the suttas, the one the Buddha intended to convey. Else, those suttas would not have survived for Ariyas to get to the deeper meaning later down the centuries.
It is the work of Dhammata in both cases – the one just in the preservation of the Dhamma in Its literal sense, the other in Its true exposition by Ariyas, both in their own time and place, and in both cases to those ‘deserving’ of it.
As to the main point under discussion – whether LISTENING is necessary to attain the Sotapanna Stage – that is still debatable. One way I see that would resolve the issue is if someone were to declare to have attained the Sotapanna Stage WITHOUT ever having listened to a single desana. (In case the doubt arises, an Ariya would not even THINK of lying, since the consequences of that will be full well known to him/her; and if he/she were NOT an Ariya, but only claiming to be one, still he/she will no doubt know what it WILL lead to. It has not escaped my notice that all so-called ‘enlightened masters’ declare only a ‘sudden bolt out of the blue’ and never any stage magga phala).
Afterthought: but…there are or may be those who are BORN Sotapannas.