Forget logic when you are talking about using CMOS as a amplifier. In this case Vin is biased at some point between logic one and logic zero to give operation in the linear region as an amplifier.
In the circuit you show, the PMOS is acting like a drain load resistor for the NMOS. The PMOS is connected so it acts like a diode, but it is a diode with significant resistance. It is that resistance that acts as a load.
If you substituted a resistor for the PMOS, then you would better see how it works.