So the exhaust gasses, exiting partilally in all directons, will try and accelerate the fuselage, and with it the complete aircraft.
Yeah, that's kinda how I read it. That the upper fuselage helped to direct the thrust down and aft. Kind of like a primitive, unmoving form of thrust vectoring.
With rockets I think they call it 'bloom', where at high altitude the gases expand outwards after they exit the nozzle bell. With the Phantom tail configuration about 1/4 of that bloom would be directed aft and downward after it leaves the engine, helping rather than being wasted.