Creepage on SMT DIP package with an NC pin
It does, but only by the width of the pin. While the pin is conductive, it is not shorted to either active pin.

Even if it were, you need to specify the standard you're applying. For instance, using the calculator at this site establishes that for IPC2221B, 800 volts requires a coated spacing of 67.7 mils, and the recommended footprint for the part establishes a 75 mil spacing.

Accepting that the floating pin simply reduces the total spacing, the footprint provides a 150 mil clearance, and IPC2221B requires 157.7 mils, which is very close.