Output Capacitor value calculation based on Transient response in Buck converter
The reason is that with the given values, Vin >> Vout.

A qualitative way of looking at this is:
When the voltage across the inductor is large (this voltage is Vin-Vout for Vunder), it takes a short time for the inductor to make up the current step. Relatively, a smaller amount of deficit energy is drained from Cout.
When the voltage across the inductor is small (this voltage is Vout for Vover), it takes a long time to make up the current step. Therefore, a greater amount of excess energy is dumped into Cout.

------ New response to comment below



The line sketched in red would be more to scale in time for Vin >> Vout. Notice the sizes of the hatched areas which represent the current deficit and the current excess which have to be made up by the capacitor. All this is consistent with the energy change of the inductor being the same.

----- New response
There are two common applications for the buck converter. One is to control the current, as in a LED driver. The other is to control the voltage, which is what you are looking at here. From the given equations, they seem to imply that the control loop makes instantaneous control decisions.

In contrast, a typical buck voltage regulator senses the output voltage, feeds that into a control loop of limited bandwidth. Limited bandwidth means control delay. With the control delay being larger than the inductor charging or discharging time, the transient load response is then dominated by the control delay. So we do see treatments of transient load response based on the control bandwidth in datasheets.

I have not worked on any power supplies for recent generation Intel processors. That is one place where this description maybe more relevant. So perhaps look there.