Hi,
VPP says the difference between the two inputs must not be more than 1.3V.
But referenced to GND both inputs may be near GND and also near VCC. (Common mode).
With differential inputs the absolute voltage level referenced to GND is not that important.
If CLK0 is at 0.8V and nCLK0 is at 0.5V, then the output is HIGH.
If CLK0 is at 1.5V but nCLK0 is at 1.8V, then the output is LOW, although now the CLK0 level is much higher than before.
See both inputs as comparator inputs. CLK0 is the noninverting input, nCLK0 is the inverting input.
Klaus