Where does it say that a pilot without an instructors rating cannot give instruction that is not required for the grant or renewal of a licence or rating.
If it does there will be a few training captains and airlines in trouble!
I think you are struggling to separate the rules that apply to airline training and the rules that apply to flight instruction in the classic sense.
In commercial diving all training is done as peer training, in recreational diving all training has to be carried out by an Instructor.