Think part of the confusion is over voltage - 4s lipo 'nominal' voltage is 14.8v, but charged, 4S is 16.8v (4.2v/cell is max voltage) and the esc rating is based on the charged voltage, not the 'nominal' voltage.

NiMh may be 1.2v nominal, but a decent cell fresh off charger is closer to 1.5v. But NiMh voltage falls so fast that an esc can get away with running something thats apparently higher voltage than what is allowed for lipo. Lipo voltage stays higher much longer than NiMh and thats part of the rating diff.