Don't confuse RPM with torque. Yes there is an RPM where the engine produces Max Torque, but that doesn't mean every time the engine is there it's at max torque. You could remove the propeller completely and reach 6500 RPM with almost no throttle opening, thats not max torque. Max torque only happens at FULL throttle, and since we don't hover at full throttle we need to think in terms of the amount of torque absorbed by the PROP at a given RPM. We could have 100 foot-pounds of torque available, but if the prop only absorbs 10 foot-pounds at 6500 thats what rolls the airframe. Forget about the engine completely, think only of the force {torque} required to spin the prop at the speed required to produce the needed thrust, then for every action there is an equal and opposite reaction.