The answer is found in the question "how much aerodynamic drag (due to vehicle speed) is there?"
If there's no velocity, and no drag, then there's no work being done to move the plane. And thus there is no thrust horsepower.
No THP = v(ft/sec) x f (thrust) / 550
IF THE ENGINE AND PROP CAN BE REPLACED BY A STATIC BRACKET, THERE IS NO THP.
Yes, the engine is producing rpm and torque (= mechanical hp), and the prop is stirring up the air, but no useful work is being done. Efficiency = zero.
BTW - don't confuse yourself by thinking of the aero drag created by the propwash as useful work!