IGh Re adequate thrust response on the approach (#55).
IIRC this is the basis of being able to fly stabilised approach - the basic control of the aircraft. The magnitude of the thrust change (N1, EPR, etc) and engine response will depend on the aircraft type, but neither the thrust change nor the response time will be ‘excessive’. Excessive is a relative term, but this covered by the certification of the aircraft i.e. the aircraft can be flown/landed “without undue skill” – satisfactory handling qualities, etc, etc. (CS 25. AC 25-7 “flight test guide for certification of transport category airplanes”)
The TC circular focuses on the energy of the aircraft and not the characteristics of the engine response. It specifically defines low energy state as being where “thrust has stabilized in the idle range”, not that the engines might not spool-up quickly. The other low energy aspects are that the airspeed is decreasing and the aircraft is at low altitude.
The low airspeed reduces the option to ‘trade’ speed for increased altitude (reducing the descent rate in the flare), nor can the altitude be traded -nose down- for speed, i.e. the aircraft is low and slow in the flare.
In these circumstances, thrust is the only means of adding energy. A meaningful change (not TOGA) may take 2-3 sec to take effect, and for an average flare time of 7 sec this might not be sufficient to correct the low energy situation.
I do not read the TC comment about ground contact as relating to a tail strike or the severity of contact, only that the aircraft may touch the runway during a late go around from low airspeed. I recall other advice that in such situations it may be necessary to accelerate to the GA (approach climb) speed (~Vref) before attempting the climb away.