Periodic Steady State voltage Buck Converter
Periodic steady state does mean normal operation (which obviously isn't a constant output voltage).  The idea is that the dynamics of the converter have settled and all startup transients have settled.

The net change in the capacitor voltage in a buck converter must be zero or else the output voltage would be moving around, which means the circuit wouldn't be working as a DC-DC converter.  So yes, I would say it is because the output DC voltage settles to be a constant (with some ripple).