There is nothing to that effect in the 2015 PADI Manual. There is no discussion of equivalencies for certified assistants.
Certified AssistantA certified assistant is a Teaching Status PADI Instructor, PADIAssistant Instructor or Active Status PADI Divemaster.
He was not talking about PADI. He is a NAUI Instructor and was talking about NAUI standards. As a PADI DM I assisted a NAUI Instructor in several classes.
This was before I decided to crossover from PADI. I was taking NAUI Tech Intro to Tech and Helitrox at the time. It was within standards for him to use me as an assistant once he verified my knowledge and skills were sufficient for his classes. Good thing I had been diving with some GUE trained guys at the time as well. And had been reading Scubaboard.