I feel like at times you don't understand why a peer sometimes can teach you something a teacher cannot... Different people have different knowledge and sometimes someone can teach through actual example rather than theory.
I used to coach swimming and I physically cannot do a whip kick/breaststroke kick, so I would demonstrate using someone who was really good at it to help teach it. Willis has been a pitching coach for a long time and knows the craft, but if you asked him if he can pitch all the pitches, he would likely say no.
Bauer and the crew that was there at the time, Kluber, Clev, Carrasco etc would work with each other on pitches since they could physically do the pitches and watch each others bullpens to help them on each others crafts.
Parker has always had a good splitter, so Stephan getting to see how one can throw a good one, is a great learning tool for a young pitcher. There is theory of teaching and there is example of teaching. One cannot do all of it since one cannot throw an example of all types of pitches.