To put it simply, the heart of any Light Logic gate is its Led and CdS photo resistor. This pair acts as a switch much as does a NPN transistor. Yes the CdS LDR does have a pronounced response curve to light but that's the fun in exploring how to work with that curve and overcome the drawbacks of circuit speed.

The whole point of Light Logic was to prove that DRL diode resistor logic was not restricted to AND and OR gate functions.