It is true that the Tipitaka cannot be expected to have ALL the details.
As one learns Dhamma and progresses, one would be able to “fill in the gaps” by oneself.
However, there are key insights that only a Buddha is able to see. For example, I have explained why a citta vithi has 17 cittas; see, “Gandhabba (Manomaya Kaya)- Introduction“.
– But an actual citta vithi can be seen only by Buddha. He explained it to Ven. Sariputta and that is how it was incorporated to the Abhidhamma Piṭaka.
– Even then it is not easy to to grasp that process, unless one is truly interested.
Whether a sotadvara citta vithi (generated by a Noble Person) is necessary to trigger the Sotapanna magga and phala citta in a listener (while listening to a discourse by a Noble Person) cannot be figured out by us.
– Note that sotadvara citta vithi means a citta vithi generated by an arammana (sound) coming through the ear.
– It may not be necessary, but if the Tipitaka says it is necessary (or discuss that particular sotadvara citta vithi), then we would have confidence in that fact.