Sounds like his idea of zero thrust isnt quite right. What he was insisting on sounds to me like a prop that is being driven by airflow and not the engine, which of course, is worse than zero thrust as you rightly point out.
How did he not fathom the difference between min rpm and feathered?
I use the debrief at the end of a check to sort stuff like that out and I find that if you ask for a diagram on a whiteboard they usually end up talking themselves round sooner or later.
If he needs proof that its the airflow driving the prop at max rpm and pwr lvr at idle in flt, ask him why that same rpm cannot be achieved on the ground when condx lvrs max, and pwr lvrs at flt idle?
Failing that, when you find the reference in a book, photocopy it and quietly leave it in his drop file for him to find.
