I agree that energy lost to turning the engine must come from the airstream, don't underestimate how much energy can be lost just to creating turbulence in the

I agree that energy lost to turning the engine must come from the airstream, don't underestimate how much energy can be lost just to creating turbulence in the airstream - after all, that's why you drop like a rock if you do a slip with full flaps. And a stalled windmilling prop will generate less turbulence than a stalled stopped prop. The discussion at your link makes a good point though - the windmilling prop might not be stalled while a stopped prop certainly is stalled, and a stalled prop obviously creates less drag.