Decoupling Capacitor (High Frequency Spike)
From my understanding, a capacitor placed at an input pin should shunt
  excess current through the capacitor proportional to the rate of
  change of voltage.


That is precisely how it works but all supply lines will have inductance and some resistance and this makes the capacitor effective in restricting the rise of the voltage spike. A capacitor on its own has no effect; it is the capacitor in conjunction with series components such as L and R that make it effective against fast transients.

However, for a continuous or long term change the capacitor is ineffective.