I think the idea is that you use registers and apply the NOR gate multiple times. In this sense the NOR gate is universal, while an OR gate nor a XOR gate are neither universal.

I.e.

  XOR(x,y) = NOR(0, NOR(NOR(x,y), NOR(NOR(x,0),NOR(y,0)))))

(maybe not the most minimal version and maybe even wrong)