Because it was originally a turbo-shaft engine I'll let the "HP" issue slide. Had it always been a turbo-jet, "HP" wouldn't be valid for a unit of measure, "thrust" would.
Horsepower is factor of torque and rotational speed. Because of the conversion from using a shaft as a means of force transmittance, there is now no rotation, no moment arm, and therefore no 'torque' and no shaft from which to determine rotation rate. Instead of pounds of force measured at some distance in feet from the driving shaft center, and some rpm of that eliminated driving shaft, there is just pounds of force.